THE CONDITIONS OF PLANT LIFE 19 



tissue above them. Plants growing in deep shade have 

 usually larger and more delicate leaves than those fully 

 exposed to the sun. 



The principal sources of plant food are carbon dioxide 

 and oxygen, obtained from the atmosphere, and water 

 with various inorganic substances in solution, usually 

 absorbed by the higher plants from the earth. When 

 the plant is completely submerged, as are many algae, 

 and a considerable number of flowering plants also, the 

 food substances dissolved in the water may be taken in 

 at almost any point. Except in a few doubtful cases 

 among the lowest plants, all food taken in must be in 

 a gaseous or fluid form. 



Where the plant is unicellular, of course this single 

 green cell must perform all the nutritive functions, and 

 is at the same time reproductive. Such a simple plant 

 consists of a single globular or oval cell surrounded by 

 a membrane of cellulose, within which is the nucleated 

 protoplasmic mass with one or more chromatophores 

 or chloroplasts. Such a cell can absorb water with 

 various food substances, including free oxygen and 

 carbon dioxide in solution.' The chromatophores, in 

 some way not clearly understood, decompose the car- 

 bon dioxide and water, and of the elements carbon, 

 hydrogen, and oxygen, manufacture the carbo-hydrates 

 upon which the protoplasm is dependent for its growth. 

 The first product of this process which can be recog- 

 nized, is usually starch, which appears in the form of 

 granules within the chloroplasts shortly after they are 

 exposed to the action of light, which, as we have seen, 

 is a necessary condition for photo-synthesis. 



As a result of the assimilation of the food absorbed, 



