BOTANY OF THE LIVING PLANT 



plant and undergo transference to some other part, the growth of 

 ^sequence modified. Such substances are now termed 

 Plant Harm they correspond to those products of the animal 



>m to which the term Hormone was originally applied For 

 imple, there is evidence that the dormancy of lateral buds is due 

 the production within the terminal bud and young leaves of a 

 hormone which p ,l.»wn t he stem and inhibits lateral bud develop- 



ment. A similar explanation is now forthcoming for the promoting 

 • ol buds on cambial activity and root formation. Chemical 

 have been discovered which simulate the effect of the 

 natural hormone By supplying such substances to the plant a 

 particular effect may be obtained more rapidly than would normally 

 be the case. Thus the formation of roots on stem-cuttings can 

 frequently be hastened by preliminary treatment with a variety of 

 chemicals (see Fig. 92). 



The particular instance of hormone-action in plants that has been most 

 in-, i still remains to be considered. Experiments suggest that the 



etching of cells in the zone of cell-elongation in shoots (pp. 142-3) is markedly 

 bed by a hormone which is chiefly produced by the tissues at the tip 

 of the shoot, and then diffuses back to the region of cell-elongation. A 

 Itate of affairs may exist in roots. These effects are of special import- 

 ance in connection with the tropic curvatures of shoots and roots (see p. 153). 



1 far most of the work on this particular hormone-effect has been carried 

 out on the young shoot or sprout of seedlings of Oat or other Grass, since 

 it provides very convenient material for experiment. Actually it is the 

 outer cylindrical sheath, or coleoptile, of the shoot that is used in these 



cperiments. This sheath encloses the developing foliage leaves. The actual 

 formation of the cells of the coleoptile is completed at an early stage, and the 

 Bobaeqnent growth of the organ, which occurs chiefly in the basal and central 

 zones, is due simply to cell-elongation. The coleoptile is an organ of limited 

 growth and is soon split open by the development of the leaves. 



cperiment shows that if the extreme tip of the organ is amputated, 

 the growth of the coleoptile rapidly diminishes to a very low level. That 

 tl. t is not merely due to injury is suggested by the observation that 



if the tip is immediately replaced in position after amputation the reducing 

 rowtfa is much less than in the absence of the tip. It appears 

 then thai the tip in some way promotes the elongation of cells in the lower 

 part of the organ, and there is convincing evidence that a growth-promoting 

 horn: 1 by the tip. For instance, a number of coleoptile tips may 



be plan "1 on a thin shed of agar-jelly for a time. If small blocks of the agar 

 are then prepared and placed on decapitated coleoptiles, the growth of these 

 will Ik- inn. h stimulated : evidently as the result of the diffusion of some 



nbstance out of the agar into the organ. If the blocks are placed asymmetri- 

 cally on the coleoptile a curvature away from the side below the agar block 

 will develop -• I , \ considerable amount of information about the 



