14 PLANT LIFE 



preserved it, but have acquired all the acces- 

 sory complications of structure that purposive 

 motion necessarily entails ? 



The answer is to be sought in the results of 

 an apparently trivial difference in structure 

 between the animals and plants which made 

 its appearance at an early period in evolution- 

 ary history. It is a difference which from 

 the start was fraught with consequences of 

 the greatest importance, and has profoundly 

 affected the entire course of development 

 in the two kingdoms respectively. Stated 

 simply, it consists essentially in this, namely, 

 that the living substance of the plant secretes 

 over its surface a skin of cellulose, or some 

 analogous substance, whilst that of the 

 animal does not. 1 



If we examine any one of the simplest 

 microscopic individuals of whose vegetable 

 'nature there can be no dispute, we shall find 

 that the protoplasm, or living substance, is 

 enclosed in a not-living skin or bag of cellulose. 

 This skin is not an indispensable structure, 

 for the living substance may, for a time at 

 least, exist without it. Even in the highest 

 plants this commonly occurs during the first 

 stages of embryonic existence, but as soon 

 as development begins the membrane is 



1 This statement is broadly true, for although cellulose 

 is not unknown in the animal kingdom it has never been 

 so arranged in the body as to affect the whole relations 

 of the animal to its physical environment as it does in 

 plants. 



