Hormone Production in Plants

The first problem I investigated in my research career was how the hormone auxin (indole-3-acetic acid) is produced in plants. See scientific papers below. When I started research on this subject, nobody knew how it was made, and years of efforts had failed to clarify the situation. As I reflected on the biochemical mechanisms by which auxin can be produced I realised that it could be a non-specific breakdown product of the amino-acid tryptophan and that it was likely to be produced in dying cells, as proteins break down, releasing tryptophan. When I started this research, in the 1960's, little attention was paid to dying cells in plants or animals. Nevertheless programmed cell death - also known as apoptosis - is now a very fashionable topic of research in cell biology.

I showed dying cells could produce auxin as a by-product of their autolysis or self-digestion. See The Production of Auxin by Autolysing Tissues

There was nothing specific about the way it was produced in plants. Auxin was also produced, for example, by autolysing yeast cells, and also by autolysing rat liver. Many of the places in which auxin is known to be produced in plants are places where cells die, for example in germinating seeds, as storage tissues breakdown. Auxin is known to be produced in developing leaves and buds, and its formation is roughly proportionate to the development of veins within the leaf. Veins contain xylem or wood cells, and when wood cells develop, the walls thicken up and the cell contains break down and are dissolved away, so cell death occurs in all young growing tissues. Perhaps the differentiating xylem cells were a source of auxin, released as they died.

I studied this question in stems in which new xylem cells were being formed as a result of cambial activity, and found that these thickening stems did indeed produce auxin. See The Production of Auxin by Tobacco Internode Tissues

I also looked at auxin production in senescent leaves. As they go yellow, the cells breakdown and sure enough I found the auxin levels increased dramatically. See Production of Auxin by Detached Leaves. There was only one supposed site of auxin production in plants in which dying cells were not present, namely the tips of coleoptiles in cereal seedlings. These sheathing structures around the seedling shoot were some of the first organs in which auxin was studied and were of particular importance in the classical literature on auxin production. However, although auxin was present in coleoptile tips, I found there was no persuasive evidence that it was made there, and found that in fact it was probably accumulating there having been carried up from the seed in the sap. See Do Coleoptile Tips Produce Auxin?

The formation of auxin in developing xylem cells in the trunks of trees as they grow would mean that a gradient of auxin would be set up across the cambium, the region of dividing cells that separates the wood from the bark. I directly measured auxin levels in the xylem cambium and young phloem cells, from the inside of the bark and showed that there was indeed such a gradient. This was one of the first chemical gradients to be characterised in either animals or plants of a chemical known to have morphogenetic effects. See Auxin in the Cambium and its Differentiating Derivatives

Since dying cells produce auxin, and since dying cells occur within all higher plants as a result of xylem differentiation this raised an evolutionary question. Had the responsiveness of plants to this cell-breakdown product, acting as a chemical signal of cell death, evolved only after cell death became an integral part of plant grown with the evolution of a vascular system? Or have plants already become sensitive to auxin before the vascular system evolved? In fact it was already known that non-vascular land plants, like mosses and liverworts are sensitive to low concentrations of auxin in the environment. They react by producing root hairs, or rhizoids. If this sensitivity had developed in response to dying cells, it would enable mosses and liverworts to produce rhizoids which increase the surface area for absorption of nutrients, in places where there was decaying organic matter, in other words when nutrients were most likely to be abundant. Is auxin really present in such situations? I examined the humus on which mosses and liverworts were growing both in the tropics and in temperate countries and found that it did in fact contain auxin in quantities sufficient to produce rhizoid formation. This suggested an evolutionary origin for the auxin responses in higher plants. First, plants evolved sensitivity to auxin as a signal of organic decay in the external environment. Later, as cell death became an integral part of plant growth the evolution of the vascular system, this hormonal-response system became internalised and auxin evolved the wide range of signalling roles that it has today. See The Occurrence and Significance of Auxin in the Substrata of Bryophytes

At the end of my time at Cambridge, I published a comprehensive review of research on production of auxin and other hormones in plants, summarising the dying-cell hypothesis. See The Production of Hormones in Higher Plants

Scientific Papers on Hormone Production In Plants

The Production of Auxin by Dying Cells

Journal of Experimental Botany (2021) Vol. 72, 2288-2300
by Rupert Sheldrake


In this review, I discuss the possibility that dying cells produce much of the auxin in vascular plants. The natural auxin, indole-3-acetic acid (IAA), is derived from tryptophan by a two-step pathway via indole pyruvic acid. The first enzymes in the pathway, tryptophan aminotransferases, have a low affinity for tryptophan and break it down only when tryptophan levels rise far above normal intracellular concentrations. Such increases occur when tryptophan is released from proteins by hydrolytic enzymes as cells autolyse and die. Many sites of auxin production are in and around dying cells: in differentiating tracheary elements; in root cap cells; in nutritive tissues that break down in developing flowers and seeds; in senescent leaves; and in wounds. Living cells also produce auxin, such as those transformed genetically by the crown gall pathogen. IAA may first have served as an exogenous indicator of the presence of nutrient-rich decomposing organic matter, stimulating the production of rhizoids in bryophytes. As cell death was internalized in bryophytes and in vascular plants, IAA may have taken on a new role as an endogenous hormone.

The Production of Hormones in Higher Plants

Biological Reviews (1973) Vol. 48, pp.509-559
by Rupert Sheldrake


1 Although much is known about the effects of plant hormones and their role in the control of growth and differentiation, little is known about the way in which hormone production is itself controlled or about the cellular sites of hormone synthesis. The literature on hormone production is discussed in this review in an attempt to shed some light on these problems.

2 The natural auxin of plants, indol-3yl-acetic acid (IAA) is produced by a wide variety of living organisms. In animals, fungi and bacteria it is formed as a minor by-product of tryptophan degradation. The pathways of its production involve either the transamination or the decarboxylation of tryptophan. The transaminase route is the more important.

3 In higher plants auxin is also produced as a minor breakdown product of tryptophan, largely via transamination. In some species decarboxylation may occur but is of minor important. Tryptophan can also be degraded by spontaneous reaction with oxidation products of certain phenols.

4 The unspecific nature of the enzymes involved in IAA production and the probable importance of spontaneous, non-enzymic reactions in the degradation of tryptophan make it unlikely that auxin production from tryptophan can be regulated with any precision at the enzymic level. The limiting factor fro auxin production is the availability of tryptophan, which in most cells is present in insufficient quantities for its degradation to occur to a significant extent. Tryptophan levels are, however, considerably elevated in cells in which net protein breakdown is taking place as a result of autolysis.

5 An indole compound, glucobrassicin, occurs in Brassica and a number of other genera. It breaks down readily to form a variety of products including indole acetonitrile, which can give rise to IAA. There is, however, no evidence to indicate that glucobrassicin is a precursor to auxin in vivo.

6 Conjugates of IAA, e.g. IAA-aspartic acid and IAA-glucose, are formed when IAA is supplied in unphysiologically high amounts to plant tissues. These and other IAA conjugates occur naturally in developing seeds and fruits. There is no persuasive evidence for the natural occurrence of IAA-protein complexes.

7 Tissues autolysing during prolonged extraction with ether produce IAA from tryptophan released by proteolysis. IAA is produced in considerable quantities by autolysing tissues in vitro.

8 During the senescence of leaves proteolysis results in elevated levels of tryptophan. Large amounts of auxin are produced by senescent leaves.

9 Coleoptile tips have a vicarious auxin economy related to IAA from the seed. These move acropetally in the xylem and accumulate at the coleoptile tip. The production of auxin in coleoptile tips involves the hydrolysis of IAA esters and the conversion of labile, as yet unidentified compounds, to IAA. There is no evidence for the de novo synthesis of IAA in coleoptiles.

10 Practically all the other sites of auxin production are sites of both meristematic activity and cell death. The production of auxin in developing anthers and fertilized ovaries takes place in the regressing nutritive tissues (tapetum, nucellus, endosperm) as the cells break down. In shoot tips, developing leaves, secondarily thickening stems, roots and developing fruits auxin is produced as a consequence of vascular differentiation; the differentiation of xylem cells and most fibres involves a complete autolysis of the cell contents; the differentiation of sieve tubes involves a partial autolysis. There is no evidence that meristematic cells produce auxin.

11 The lysis ad digestion of cells infected with fungi and bacteria results in elevated tryptophan levels and the production of auxin. Viral infections reduce the levels of tryptophan and are associated with reduced levels of auxin.

12 Crown-gall tissues produce auxin. It is suggested that the crown-gall disease may involve at any given time the death of a minority of the cells which produce auxin and other hormones as they autolyse; the other cells grow and divide in response to the hormones.

13 Auxin is produced in soils, particularly those rich in decaying organic matter, by micro-organisms. This environmental auxin may be important for the growth of roots.

14 There is no convincing evidence that auxin is a hormone in non-vascular plants. The induction of rhizoids in liverworts by low concentrations of auxin can be explained as a response to environmental auxin.

15 Abscisic acid is synthesized from mevalonic acid in living cells. It is possible that under certain circumstances, abscisic acid or closely related compounds are formed by the oxidation of carotenoids.

16 The sites of gibberellin production are sites of cell death. It is possible that precursors of gibberellins, such as kaurene, are oxidized to gibberellins when cells die.

17 Cytokinins are present in transfer-RNA (tRNA) of animals, fungi, bacteria and higher plants. They are probably formed in plants by the hydrolysis of tRNA in autolysing cells. There is evidence that they are also formed in living cells in root tips.

18 Ethylene is produced in senescent, dying or damaged cells by the breakdown of methionine.

19 It was shown many years ago that wounded and damaged cells produced substances which stimulate cell division. It now seems likely that the production of wound hormones and the normal production of hormones as a consequence of cell death are two aspects of the same phenomenon. Wounded cells can produce auxin, gibberellins, cytokinins and ethylene.

20 The control of hormone production in living cells is a biochemical problem which remains unsolved. The control of production of hormones formed as a consequence of cell death depends on the control of cell death itself. Cell death is controlled by hormones which are themselves produced as a consequence of cell death.

21 In spite of the fact that dying cells are present in all vascular plants, in all wounded and infected tissues, in certain differentiating tissues in animals, in cancerous tumours and in developing animal embryos, the biochemistry of cell death is a subject which has been almost completely ignored. Dying cells are an important source of hormones in plants; some of the many substances released by dying cells may also be of physiological significance in animals.

Do Coleoptile Tips Produce Auxin?

New Phytol. (1973) Vol. 72, 433-447
by Rupert Sheldrake


A re-examination of the evidence for auxin production by coleoptile tips reveals that it is not conclusive and that several important problems remain unresolved. The possibility that auxin and auxin precursors move acropetally in the xylem was tested by analysing guttation fluid from intact coleoptiles, decapitated coleoptiles and primary leaves of Avena sativa. In all cases two zones of auxin activity were detected on chromatograms of the acidic ether-soluble fraction, one of which corresponded to the Rf of indol-3-yl acetic acid (IAA). Similar auxin activity was found in guttation fluid from seedlings of Zea mays, Triticum aestivum and Hordeum vulgare. Evidence that guttation fluid also contains alkali-labile auxin complexes was obtained. Experiments on the movement of dyes and radioactive IAA introduced into the xylem of transpiring or guttating coleoptiles showed that these substances accumulate at the tip of the coleoptile, or at the apical region of decapitated coleoptiles. The hypothesis that IAA and 'inactive' auxins move acropetally in the xylem from the seed to the coleoptile tip where they accumulate and where the 'inactive auxins' can be converted to IAA is shown to be consistent with the classical work on coleoptiles; it can also explain the autonomous curvature of coleoptiles and the influence of the roots on the auxin contect of coleoptile tips. An analogous accumulation of auxin probably occurs at the tips of primary leaves. The anomalous auxin economy of coleoptile tips is discussed.

Auxin in the Cambium and its Differentiating Derivatives

Journal of Experimental Botany (1971) Vol. 22, 735-740
by Rupert Sheldrake


Cambium and differentiating xylem and phloem tissues from the trunks of trees of Acer pseudoplatanus L., Fraxinus excelsior L., and Populus tremula L. were extracted with ether and tested for auxin, which was found on chromatograms of the acidic fraction at an Rf corresponding to that of indol-3yl-acetic acid in five solvent systems. In addition, small amounts of auxin with a higher Rf in ammoniacal isopropanol were found in phloem samples. The amounts of auxin were greatest in xylem samples, less in the cambium, and least in phloem. The differences, which cannot be explained in terms of differential losses during extraction and purification, suggest that auxin is actually formed in differentiating xylem tissue. The significance of these results is discussed.

The Occurrence and Significance of Auxin in the Substrata of Bryophytes

New Phytologist (1971) Vol. 70, 519-526
by Rupert Sheldrake


Auxin was detected in samples of substrata supporting bryophytes in a variety of locations in both Britain and Malaya. Activity occurred on chromatograms at zones corresponding to the Rf of indole acetic acid. The range of concentrations found, 0.4-10.4ug/1, probably represents a two-to five-fold underestimate due to losses during extraction and purification. The amounts of auxin in samples of soil on which bryophytes were not growing were within the same range. The importance of this environmental auxin for the induction of rhizoids in liverworts and for roots of higher plants is discussed.

The Production of Auxin by Autolysing Tissues

Planta (1968) Vol. 80, 227-236
by Rupert Sheldrake and D.H. Northcote


Autolysing plant tissues are known to produce auxin when extracted with ether. It has been shown that autolysing plant, yeast and rat liver tissues produce auxin in vitro; this suggests that relatively unspecific mechanisms are involved. Furthermore, sterile plant and animal tissues which have been killed by freezing and thawing induce nodules of differentiated cells in a previously undifferentiated callus of Phaseolus vulgaris. The callus tissue is known to differentiate in response to applied gradients of auxin. Plant and animal tissues killed by boiling were considerably less effective in inducing differentiation in the tissue. The evidence indicates that auxin is a normal product of autolysing cells. It is suggested that dying cells are an important source of auxin in the plant.

Production of Auxin by Detached Leaves

Nature (1968) Vol. 217, 195
by Rupert Sheldrake


In senescent leaves proteins are hydrolysed to amino-acids and peptides, which might be expected to release protein-bound auxin and also to provide considerable amounts of trypotophan which can be converted by many plant tissues to the auxin indolyl-3-acetic acid (IAA). We have therefore investigated the concentrations of auxin in senescent leaves.

Mature trifoliate leaves from plant of Phaseolus vulgaris and leaves from young plants (2-3 weeks old) of Avena sativa were detached and placed with their petioles or bases in distilled water in the dark at 25° C. In these conditions, the leaves become senescent and turn yellow. Samples were taken at various times (at intervals of 1 or 2 days), weighed and stored in the deep freeze until they were extracted with peroxide-free ether for 3 h at 0° C. The ether extract was partitioned and the acidic fraction was run on paper chromatograms with isopropanol : ammonia : water (8:2:1 v/v). The zone corresponding to IAA was eluted and the auxin was estimated using an Avena coleoptile straight growth bioassay. The amounts of auxin extracted from the leaves at various times are shown in Figs. 1 and 2.

It can be seen that in both cases there is a large increase in the amount of auxin present over a period of 6 days. The amounts measured represent the resultant of auxin production and auxin destruction: in the case of Avena, after about the fourth day the rate of destruction exceeds the rate of production. The fall in total auxin was observed in each of six experiments.

The level of auxin in leaves and petioles is involved in the control of abscission so the production of auxin by senescent leaves, if it is a general phenomenon, may be an important factor which so far has been overlooked.

The Production of Auxin by Tobacco Internode Tissues

New Phytologist (1968) Vol. 67, 1-13
by Rupert Sheldrake and D. Northcote


The formation of callus at the basal end of tobacco internode tissues cultured on a basic medium has been used as an indication of the presence of auxin within the tissues. It has been shown in this way that sections of internode are capable of producing auxin. This production of auxin is related to the continued activity of the vascular cambium. If cambial activity and vascular differentiation are eliminated, auxin is no longer produced. When tissues in which cambial activity and vascular differentiation are taking place are cultured on a medium containing an inhibitor of polar auxin transport, tri-iodo benzoic acid, serried ranks of xylem tracheids are formed. It is suggested that auxin is produced as a consequence of xylem differentiation and the observations reported in this paper are interpreted in the light of this hypothesis. It is also suggested that kinins may be produced as a result of xylem and phloem differentiation, and the possibility that autolysing cells are a major source of both auxins and kinins in the plant is discussed.

Auxin Transport in Plants

In plants auxin is transported from the shoot tips towards the root tips by the polar auxin transport system. I investigated which tissues were most involved in this transport process, and whether the polarity of stems could be reversed: I found it could not be. With my colleague Philip Rubery, I worked out the cellular basis of polar auxin transport. Our hypothesis, the so-called chemiosmotic hypothesis, was subsequently confirmed and is now generally accepted. We predicted the existence of auxin efflux carrier proteins preferentially located at the basal end of cells. These proteins were identified in the twenty-first century, and are now called PIN proteins; they are an important focus for contemporary research on plant development.

Scientific Papers on Auxin Transport In Plants

Effects of Osmotic Stress on Polar Auxin Transport in Avena Mesocotyl Sections

Planta (1979) Vol. 145, 113-117
by Rupert Sheldrake


Segments of mesocotyls of Avena sativa L. transported (1-14C) indol-3yl-acetic acid (IAA)with strictly basipetal polarity. Treatment of the segments with solutions of sorbitol caused a striking increase in basipetal auxin transport, which was greatest at concentrations around 0.5M. Similar effects were observed with mannitol or quebrachitol as osmotica, but with glucose or sucrose the increases were smaller. Polar transport was still detectable in segments treated with 1.2M sorbitol. The effects of osmotic stress on the polar transport of auxin were reversible, but treatment with sorbital solutions more concentrated than 0.5M reduced the subsequent ability of mesocotyl segments to grow in response to IAA. The increased transport of auxin in the osmotically stressed segments could not be explained in terms of an increased uptake from donor blocks. The velocity of transport declined with higher concentrations of osmoticum. The reasons for the enhancement of auxin transport by osmotic stress are not known.

Carrier-mediated Auxin Transport

Planta (1974) Vol. 118, 101-121
by P.H. Rubery and R. Sheldrake


Auxin (IAA) transport was investigated using crown gall suspension tissue culture cells. We have shown that auxin can cross the plasmalemma both by transport of IAA anions on a saturable carrier and by passive (not carrier-mediated) diffusion of the lipid-soluble undissociated IAA molecules (pK=4.7). The pH optimum of the carrier for auxin influx is about pH6 and it is half-saturated by auxin concentrations in the region of a 1-5u-M. We found that the synthetic auxin, 2,4D specifically inhibited carrier-mediated IAA anion influx, and possibly also efflux. Other lipid-soluble weak acids which are not auxins, such as 3,4-dichlorobenzoic acid, had no effect on auxin transport. By contrast, we found that TIBA, an inhibitor of polar auxin transport in intact tissue inhibited only the carrier-mediated efflux of IAA.

When the pH outside the cells is maintained below that of the cytoplasm (pH7), auxin can be accumulated by the cells: In the initial phase of uptake, the direction of the auxin concentration gradient allows both passive carrier-mediated anion influx (inhibited by 2,4D) and a passive diffusion of undissociated acid molecules into the cells. Once inside the cytoplasm, the undissociated molecules ionise, producing IAA anions, to a greater extent than in the more acidic extra-cellular environment. Uptake by passive diffusion continues as long as the extra-cellular concentration of undissociated acid remains higher than its intra-cellular concentration. Thus, the direction of the auxin anion concentration gradient is reversed after a short period of uptake and auxin accumulates within the cells. The carrier is now able to mediate passive IAA anion efflux (inhibited by TIBA) down this concentration gradient even though net uptake still proceeds because the carrier is saturable whereas passive diffusion is not.

Auxin 'secretion' from cells is regarded as a critical step in polar auxin transport. The evidence which we present is consistent with the view that auxin 'secretion' depends on a passive carrier-mediated efflux of auxin anions which accumulate within the cells when the extra-cellular pH is below that of the cytoplasm. The implications of this view for theories of polar auxin transport are discussed.

The Polarity of Auxin Transport in Inverted Cuttings

New Phytol (1974) Vol. 74, 637-642
by Rupert Sheldrake


The original, basipetal polarity of auxin transport persisted in the stems of inverted cuttings of Tagetes, tomato and tobacco in spite of the reversal of the relative positions of the roots and shoots. No significant acropetal auxin transport could be detected even after four months growth. These results indicate that the polarity of newly formed cells in secondarily thickening internodes is determined by the existing polarity of auxin transport within the tissues.

Auxin Transport in Secondary Tissues

Journal of Experimental Botany(1973 Feb) Vol. 24, No. 78, 87-96
by Rupert Sheldrake


Auxin transport was investigated in excised stem segments of Nicotiana tabacum L. by the agar block technique using (I-14C) indol-3yl-acetic acid (IAA). The ability of the stems to transport auxin basipetally increased as secondary development proceeded; by contrast the ability of the pith to transport auxin declined with age. By separation of the stem tissues it was shown that the great majority of auxin transport took place in cells associated with the internal phloem and in cells close to the cambium; in both cases similar velocities of transport were found (c 5.0 mm h-1 at 22°C). The effects of osmotic gradients on auxin transport through the internal phloem were investigated. IAA was found by chromatography to account for practically all the radioactivity in receiver blocks and ether extracts of stem segments. The significance of these results is discussed.

Effect of pH and Surface Charge on Cell Uptake of Auxin

Nature New Biology (1973) Vol. 244, 285-288
by P.H. Rubery and A.R. Sheldrake


The uptake of the auxin indol-3-yl acetic acid (IAA) into plant cells is of interest not only because this compound is a hormone, but also because its movement across the plasma membrane is probably involved in the polar transport of auxin. The plasma membrane contains auxin binding sites and may be a primary site of hormone action.

IAA partitions into non-polar solvents from acidified aqueous solutions because the undissociated acid is more soluble in such lipid solvents than in water. There is known to be a passive, non-metabolic component of the uptake of IAA and of the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) into plant tissue which has been ascribed to the diffusion of the undissociated acid across the plama membrane. A carrier-mediated mechanism for auxin anion uptake is also possible but has not been conclusively demonstrated.

Uptake by the diffusion mechanism is linearly related to the concentration of the undissociated acid which is a function of the acid's pK and the pH of the incubation medium. If the pH of the medium is lower than that of the cells, the cells accumulate weak acid; the equation requires that the concentration of undissociated acid should be the same in each compartment. Thus the relation between the initial rate of uptake and pH should resemble a dissociation curve with a midpoint at the pK of the weak acid. This prediction is realized for the uptake of benzoic acid (pK=4.2) by yeast but not by the bacterium Proteus vularis, when, although the curve is still that of a dissociation, its midpoint is displaced by 1 pH unit above the pK of benzoic acid. Such displacement seems fairly widespread. By collating data from ninety experiments on pH dependence of biological effects of weak acids, a composite curve is obtained relating pH to log concentration of acid required to give a standard response; the midpoint of the curve is at a higher pH than the pK. Data on IAA and 2,4-D uptake reveal a similar effect. Here we suggest an explanation of this displacement which may be of general biological significance.

Polar Auxin Transport in Leaves of Monocotyledons

Nature (1972) Vol. 238, 352-353
by Rupert Sheldrake


Almost nothing is known about the establishment of cellular polarity underlying the polar auxin transport system of higher plants. Osborne has suggested that the apical ends of cells derived from an apical meristem by sequential divisions are younger than the basal ends: their polarity and the basipetal transport of auxin are due to this age difference. Sachs in his work on regenerating vascular strands has found that gradients of auxin may be responsible for establishing the cellular polarity and the subsequent transport of auxin in the direction of the initial gradient. Shoot tips and expanding dicot leaves contain relatively high levels of auxin. The basipetal polarity of auxin transport in petioles and stems is therefore associated with basipetal auxin gradients. In grass coleoptiles the greatest amounts of auxin are found at the tip, where basipetal auxin transport is also associated with basipetal auxin gradients."

In monocot leaves which grow by a basal intercalary meristem, the pattern of cell division and of auxin distribution is more or less the reverse of that found in shoot tips. Sequential divisions of the basal meristem presumably make younger the basal ends of cells; and in growing monocot leaves the greatest amounts of auxin are found at the base. The polarity of auxin transport in monocot leaves is therefore of considerable interest.

Hertel and Leopold reported that in the primary leaf of Zea mais, auxin transport was basipetal. No other references to auxin transport in monocot leaves are available and I therefore tested the leaves of a number of species. In every case auxin transport was basipetal.

In leaves of young plants of Avena sativa, basipetal auxin transport took place across the meristematic region at the base of the leaf and also in the leaf sheath, which grows by a basal meristem. Plants germinated and grown in darkness yielded similar results. Less auxin transport was found near the leaf tip than in the younger, more basal parts of the leaf and younger leaves had a greater ability to transport auxin than older leaves. A decline in the ability of cells to transport auxin as they grow older has been observed in a number of other species and tissues.

Rupert's research reports as Rosenheim Research Fellow

Royal Society Yearbooks for 1971, 1972 and 1973