GROWTH, IRRITABILITY AND MOVEMENT 151 



chemical nature of this growth-promoting hormone, which is known as 

 Auxin, has been obtained, and a number of other substances, natural 

 and synthetic, have been found to stimulate the growth of the coleoptile 

 much like Auxin itself. The man- 

 ner in which cell-elongation is 

 affected is not yet clear, though 

 there is evidence that the hormone 

 increases the plasticity of the cell- 

 walls (seep. 142). Similar experiments 

 to those described for the coleoptile Fig. 93. 



have been carried out with shoots of Curvatures in decapitated Oat coleoptiles 



resulting from the application of agar blocks 

 some dicotyledonous plants with like containing a substance of similar action to Auxin. 



results • thus the stem anex annears The first leaf was partly pulled , °, ut of th * cole / 

 results . tnus tne stem apex appears optile at the commencem ent of the experiment, 



to produce a hormone which promotes and in each plant it is seen projecting beyond the 

 „ , . top of the amputated coleoptile. (From Thimann 



cell-elongation in lower regions of the an d Went, 1937.) 



stem. 



The position with regard to cell-elongation in the root is rather obscure. 



Parallel experiments to those described above suggest that root elongation 



is somewhat accelerated by amputation of the tip, and that auxin and other 



substances which promote the growth of shoots retard that of roots. The 



explanation of these observations must await further investigation. 



3. Physiological Aspects of Reproduction. 



So far we have considered chiefly the vegetative phase of the 

 growth of plants, in which the plant body, consisting of root and 

 shoot systems, is built up. At the same time, many of the remarks 

 in the preceding section apply to growth processes as a whole. Sooner 

 or later in the growth and development of the higher plant repro- 

 ductive structures are produced, first the flowers and from these the 

 fruits and seeds. The manner of development and the structure of 

 these organs are considered in later Chapters: at present we are 

 primarily concerned with the study of the factors that induce the 

 formation of reproductive organs. In the case of annual and biennial 

 plants the origination of reproductive structures happens but once in 

 the life-cycle, and after their formation the plant dies. There is 

 some connection between the events, for if flowering and fruiting 

 are prevented the life of an annual plant can often be prolonged 

 considerably. In perennial plants, however, there is normally an 

 annual production of flowers, though the first flowering may not 

 occur until the plant has attained a considerable age. Thus the 

 first flowering in the oak may be delayed until the tree is forty years 

 old. 



Much attention has been paid to the study of the conditions that 

 affect flowering and fruiting. Illumination is again important. 



