30 GENERAL EVOLUTION. 



the subject is purely hypothetical, however probable, but the as- 

 pect of the discussion will be materially brightened to the reader, 

 if he have previously adopted, with the author, the principle that 

 evolution has been the mode of origin of the present life-forms of 

 the earth. 



Reverting now to those beings which are either in part or en- 

 tirely destitute of the power of determining these movements, as 

 plants, various causes present themselves as modifiers of the polar 

 or centripetal activity of growth-force. In the case of a plant, the 

 medium in which it grows modifies the result ; for example, the 

 downward growing axis or root differs much from the upward 

 aerial portion or stem. Aquatic plants, supported beneath or on 

 the surface of the water, lack the strong stems and braces neces- 

 sary to air-growing plants ; while of the latter, those with weak 

 stems develop tendrils and other supports. In these cases physical 

 laws have been the guides of growth-force. In the case of bright 

 colors, which we know to be impossible in vegetation without sun- 

 light, the influence is chemical. 



The first physical law is, that growth-force, uninfluenced by 

 inherited peculiarity,* or any stronger influence locating a nutri- 

 tive fluid, must develop extent in the direction of least resistance, 

 and density on the side of greatest resistance, when not too great. 

 The illustration of this statement would be that a globular mass of 

 cells brought to the point of Junction of two media, as water and 

 earth or air and earth, elongates in the direction of the medium pre- 

 senting the least resistance, i. e., air. Thus a longitudinal develop- 

 ment would originate from a centrifugal, and a repetition of the 

 same process would produce branches. The reason why repetition 

 should appear along the sides instead of as continued prolongation 

 of the axis, arises probably from the difficulty of conveying fluid 

 nutritive material far from the base as a source of supply, and the 

 occurrence of various mechanical obstacles easy to be conceived. 

 In low plants, where nutrition is absorbed by all parts of the axis, 

 which branch, as the subterranean parts of fungi {myceliwn), dif- 

 ference of local supply would produce an effect. In higher plants, 

 where fluid nutriment is only introduced at one point (the root), 

 and conveyed by special layers of cells or tubes, the difficulty of 

 maintaining supply at a distance from its source would encourage 

 lateral repetition or branching, f 



* This is supposed to be due to atomic peculiarity of cell-substance. 



f The great power exerted in a given direction by growth-force due to gradual 



