236 PHYSIOLOGICAL DEVELOPMENT. 



when one of tlieni is detached. Whichever side falls lower- 

 most, however, presently begins to send out rootlets, while the 

 uppermost side begins to assume those characters which 

 distinguish the face of the frond. When this differentiation 

 has commenced, the tendency to its complete establishment 

 becomes more and more decided ; as is proved by the fact 

 that if the positions of the surfaces be altered, the gemmule 

 bends itself so as to re-adjust them : the change towards 

 equilibrium with environing forces having been once set up, 

 there is acquired, as it w^ere, an increasing momentum which 

 resists anj^ counter-change. But the evidence shows that 

 at the outset, the relations to earth and air alone deter- 

 mine the differentiation of the under surface from the 

 upper. The experiences of the gardener, multi- 



plying his plants by cuttings and layers, constitute another 

 class of evidences not to be omitted : they are commonplace 

 but instructive examples of physiological differentiation. 

 While circumstanced as it usually is, the cambium of each 

 branch in a Phaenogam continues to perform its ordinary 

 function— transforming itself on its outer side into the 

 cortical substances, and on its inner side into vascular and 

 woody tissues. But change the conditions to those which 

 the underground part of the plant is exposed to, and there 

 begins another differentiation resulting in underground struc- 

 tures. Contact with water often suffices alone to produce 

 this result, as in the branches of some trees when they droop 

 into a pool, or as occasionally with a cutting placed in a 

 bottle of water ; and when the light is excluded by im- 

 bedding the end of the cutting, or the middle of the still- 

 attached, branch, in the earth, this production of tissues 

 adapted, to the function of absorbing moisture and mineral 

 constituents proceeds still more readily. With such cases 

 may be grouped those in which this development of under- 

 ground organs by an above-ground tissue, is not excep- 

 tional but habitual. Creeping plants furnish good illus- 

 trations. From the shoots of the Ground-Ivy, rootlets ar« 



