THE OUTER TISSUES OF PLANTS. 253 



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 were, an increasing momentum which 

 resists any 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 meristematic tissue 

 of each branch in a Phaenogam continues to perform its 

 ordinary function — regularly producing on its outer side the 

 cortical substances, and on its inner side the 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 are 

 put out into the soil in a manner differing but little from 

 that in which they are put out by an imbedded layer; save 



