GROWTH. 



341 



zone I being the lowest and zone IX the uppermost : the figures give the 

 increment in length of each zone for 24 hours. 



Passing now to the properties of growing organs, the one 

 which is of the greatest physiological importance is that of 

 responding to the action of stimuli. Stimuli of the most various 

 kinds, that of light, of gravitation, of electricity, of contact of 

 foreign bodies, etc., produce modifications in the turgidity 

 of the growing cells, which, when rendered permanent by 

 growth, find their expression in curvatures and other altera- 

 tions in the form and direction of growth of the organs, giving 

 rise to the phenomena which are included under the terms 

 heliotropism and geotropism, to the twining of tendrils, etc. ; 

 these phenomena we will study in subsequent lectures. In 

 endeavouring to account for the ready response of growing 

 organs to the action of these various stimuli, it is usually 

 ascribed to an especial irritability of their protoplasm. But 

 we must not overlook the greater mechanical facilities for 

 such a response which young cells possess as compared with 

 mature cells ; we can easily understand that the form of a 

 young cell with a relatively extensible cell-wall will be 

 much more obviously affected by changes induced in its 

 protoplasm, than that of an old cell in which the cell-wall is 

 relatively inextensible. 



A few words may be devoted to the purely physical pro- 

 perties of growing organs. It appears, from the observations 

 of Sachs, that growing internodes are highly extensible but 

 very imperfectly elastic. They are therefore very flexible : 

 if they are strongly bent they retain a permanent curvature, 



