18 PLANT LIFE 



and nutriment the Chlamydomonas plant 

 rapidly multiplies in a vegetative or non- 

 sexual manner. This is brought about by 

 the division of the protoplasmic contents, 

 within the membrane, into a number of 

 smaller lumps, each of which becomes a small 

 image of the original parent. They are 

 commonly two, four, or eight in number, and 

 finally escape from the ruptured membrane of 

 the " mother cell " into the surrounding water 

 where they grow, and may give rise in their 

 turn to new individuals. 



Now such a plant as Chlamydomonas is 

 relatively very simple, and yet it already 

 exhibits the most striking characters that 

 distinguish the majority of plants. 



In the first place its living substance is 

 enclosed in a membrane, and in the second 

 its protoplasm contains a green chloroplast. 

 In order to grow it must clearly obtain food, 

 but the presence of a membrane precludes 

 it from acquiring any except such as is already 

 dissolved in the water. No solid particles 

 can . pass the membrane and so reach the 

 protoplasm, but water and substances dis- 

 solved in it will readily do so. The whole 

 of the mineral food substances, and such 

 gases as oxygen and carbon dioxide, reach 

 the protoplasm in this, and only in this way. 



But this external membrane not only 

 limits the nature of the food-supply, but as 

 the size and complexity of the body increase, 

 it continues more and more to restrict the kind 



