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, 
