VII GROWTH OF SPECIFIC ORGANS 8oi 



Dicotyledonous leaves give no such marked response, and in most cases the very 

 marked elongation of the stem which gibberellin brings about (Brian and Hem- 

 ming, 1955) is accompanied by some reduction in leaf size, as well as by reduction 

 in the chlorophyll density. Work in the writer's laboratory has shown that when 

 the fronds of the duckweed Lemna are caused to expand by floating on gibberellic 

 acid solution the chlorophyll density decreases so that the total chlorophyll per 

 culture does not rise proportionately to the growth. With plants in soil this phe- 

 nomenon can apparently be prevented by extra fertilization, and as a result some 

 real, though small, increase in leaf area does result. On this account it is just 

 possible that gibberellin may have some agricultural application for leafy crops. 

 Root crops, too, whose yield has been shown to be strictly dependent on their 

 leaf surface (Watson, 1947) may prove to be benefited. 



When the leaf becomes old, protein hydrolysis commonly occurs, leading to 

 loss and fading of chloroplasts, migration of soluble ions out of the leaf, and other 

 changes. It then falls off. This last process, "abscission," results from the division 

 of cells at the base of the petiole (and sometimes also at the base of the blade) to 

 form a line of new cell walls which crosses from one side to the other (see Plate 67 

 of Esau, 1953). These new walls differ from normal walls, or perhaps they contain 

 pectinase, for they soon separate from one another, thovigh the cells on either side 

 remain intact. The separation eventually leaves the leaf hanging only by the con- 

 ducting fibers, which soon break off, and it falls. 



Almost the only fact that is understood about abscission is that it is inhibited by 

 auxin and by conditions which promote auxin formation in the leaf. It is promoted 

 by drought, ethylene and various chemical defoliants, as well as by a variety of 

 injurious circumstances (see review by Addicott and Lynch, 1955). It is even, to a 

 small extent, promoted by the presence of auxin in the stem below the leaf. 

 Evidently the normal abscission of old leaves is a consequence of the cessation of 

 auxin formation, and it was mentioned that auxin production in the leaf decreases 

 with advancing age. In at least one fungus disease the leaf fall which results has 

 been ascribed to auxin destruction by the fungus (Sequeira and Steeves, 1954). 

 A powerful enzyme for this, highly specific for lAA, is excreted externally by the 

 fungus mycelium (Ray and Thimann, 1956). It has been suggested that the deter- 

 mining factor in abscission is not merely the auxin coming from the leaf but 

 rather the difference between this and the auxin in the stem, i.e. the auxin gradient 

 across the abscission zone (Addicott and Lynch, 1955; Jacobs, 1955). It is difficult 

 to conceive how cell division can be inhibited by a gradient. Direct inhibition by 

 auxin would be a simpler concept, and it is therefore important that if very low 

 auxin concentrations, such as those reaching the abscission zone from the stem, 

 would slightly promote the process (Biggs and Leopold, 1957). 



{b) Growth of roots 



The apex of the root differs from that of the shoot in two respects : ( i ) the meri- 

 stem is not quite at the tip, but continually produces a small group of cells, the 

 root-cap, distal to itself, and (2) no outgrowths comparable to leaf primordia arise 

 on its flanks. Lateral roots are indeed formed, but they appear at a considerable 

 distance from the apex (see below). This second characteristic makes root develop- 



Lilerature p. 816 



