HORMONES 2ii 



HORMONES. 



The study of regeneration, correlation, polarity and 

 cognate subjects * leads to conclusions in some respects 

 indefinite in that no tangible factor is discoverable that 

 will account for the beginning or for the control of certain 

 phenomena. A cambium cell divides ; the daughter cell 

 destined to become a permanent tissue element will develop 

 into a phloem element if cut off on one side and into a xylem 

 element if cut off on the opposite side ; what is it that deter- 

 mines the fate of the cell ? The leader of a spruce is negatively 

 geotropic, the lateral branches are diageotropic ; if the leader 

 is removed, a lateral branch from the topmost whorl will 

 change its habit, become negatively geotropic and carry 

 on the functions of the leader. Why must the leader be 

 removed before a change in tone of a plagiotropic shoot can 

 be effected ? The primordium of a lateral bud is laid down, 

 and, apparently, all conditions are favourable for development, 

 yet the bud remains dormant until the apex of the main 

 shoot is removed, then the bud will immediately start its 

 development. It is true that often the diversion of food 

 will account for the subsequent phenomena, but in other 

 instances such an explanation is inadequate and in such 

 examples the question is : What is it that presses the trigger ? 



The subject properly is beyond the scope of an introduction 

 to the physiology of metabolism of plants, but the introduction 

 is desirable even though the acquaintance be not cultivated. 



For long it has been known that very small quantities of 

 various materials act as powerful stimulants ; the extra- 

 ordinary effect of minute traces of zinc on the growth of 

 moulds may be instanced. Animal physiologists recognize 

 the effect of traces of substances in stimulating various 

 activities, especially those associated with secretion. These 

 substances are produced in one organ and stimulate another 

 organ to which they are conveyed by the blood. Hence the 



* See Bohn : " Compt. rend. Soc. biol.," 1918, 81, 220. Farmer: 

 "New Phyt.," 1903, 2, 193, 217. Goebel : "Biol. Zentrlbl.," 1916, 36, 

 193. Lang: " Brit. Ass. Rep.," 1915. 7 QI - Loeb : " Bot. Gaz.," 1918, 

 65, 150 ; " Journ. Gen. Physiol.," 1919, I, 337. 



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