30 EVOLUTION OF PLANTS 



the carbon is directly available only for those plants 

 which possess chlorophyll ; and where the plant is desti- 

 tute of this, as in most saprophytes and parasites, it 

 must obtain its carbon from organic compounds. All 

 plants breathe by taking in free oxygen and giving off 

 carbon dioxide. 



Water is essential for the manifestation of life, but 

 the amount varies greatly in different plants as does the 

 temperature at which they will grow. 



Light is necessary to all green plants, and to most 

 others as well, but among these, the optimum illumina- 

 tion is extremely variable. 



We see, in passing from the simple unicellular plants, 

 where all the functions are performed by a single cell, 

 how there is a gradual division of labor, first in a 

 separation of the vegetative and reproductive cells, and 

 later a further specialization of both vegetative and 

 reproductive functions, which reaches its highest ex- 

 pression in the seed plants where there are special com- 

 plicated organs — leaves — for carbon assimilation, an 

 extensive system of roots for attachment and absorp- 

 tion of food from the soil, extremely modified tissues 

 for conduction, storing of food, and other functions ; 

 and finally extraordinarily varied structures — the 

 flowers — connected with the reproduction of the plant 

 and the distribution of seeds. 



The power of independent locomotion is confined to a 

 small number of the lower plants, or to certain repro- 

 ductive cells of higher ones ; but all plants exhibit more 

 or less marked movements which may be spontaneous, 

 or influenced by various external agencies, such as light 

 and gravity. 



