THE STIMULATION OF PLANT GROWTH. 



19 



of weak solutions of the strong alkalies — potash and soda. Most 

 salts cannot penetrate into the interior of the grain. The sugars also 

 cannot get through the protecting layer. But many substances 

 can get in; weak acids, such as acetic and butyric acid; weak alkalies, 

 such as ammonia; a few salts, such as mercuric chloride (corrosive 

 sublimate); and alcohol, ether, acetic ether, and inany other compounds 

 similar to these. Professor Brown's experiments bring out the fact 

 that none of the substances normally produced during germination 

 can escape from the grain so long as the protecting membrane remains 

 uninjured; and that, in like manner, the salts that are normally 

 present in soil water cannot enter; in fact, that little but water can 

 pass into or out from the seed under normal conditions during ger- 

 mination. 



Experiments that my son, Dr. E. F. Armstrong, and I have carried 

 out with Laurel and Aucuba leaves,"^ as well as with other plants, show 

 that plants generally are provided with differential septa (membranes 

 capable of distinguishing or differentiating substances) through which 

 water can pass at all times, but which are not permeable by the 

 soluble materials within the plant tissues, so that rain has no effect 

 in dissolving out substances from the leaves. Substances which 

 enter the barley grain also enter the leaf, however. In the case of 

 leaves provided with cuticularized membranes, apparently entry is 

 effected primarily only through the stomata. 



We have proposed to apply the term Hormones to all substances 

 which can penetrate the differential septa with which plants are pro- 

 vided; probably on gaining an entry, such substances produce dis- 

 turbances within the cells into which they penetrate by setting 

 enzymes in action. 



The effects shown with Laurel and Aucuba leaves are those of over- 

 stimulation and the leaf is killed ; by using only a minute proportion 

 of the hormone, change may be provoked without killing the tissue. 



[In proof of this statement, a set of slides was shown illustrating 

 the normal process of development of the sea-urchin following the 

 entry of the spermatazoon into the ovum ; also another set, showing 

 the precisely similar series of changes Professor Jacques Loeb has 

 succeeded in observing which are induced in the entire absence of the 

 male element by exposing the freshly laid eggs, during a brief period, 

 in a solution containing a minute proportion of butyric acid.] 



In the case of the plant there are apparently two periods to be 

 distinguished — that in which assimilation takes place under the influ- 

 ence of light and that in which growth takes place at the expense of 

 the materials previously laid down : the latter probably is the period 

 during which stimulation is necessary, that in which enzymes are 

 brought into action as simplifying agents and the products of their 



* ' The function of hormones in stimulating enzymic change in relation to 

 necrosis, and the phenomena of degenerative and regenerative change in living 

 structures,' Proceedings of the Royal Society (1910), B, vol. 82, p. 588. 'The 

 function of hormones in regulating metaboHsm,' Annals of Botany, 1911, 

 vol. 25, p. 507. s s / J> , 



c 2 



