CHAPTER XIV 
INFLUENCE OF ENVIRONMENT 
THE conditions for normal plant-growth are light, 
heat, moisture, and certain food constituents, including 
carbon dioxide, oxygen, and some nitrogen compounds. 
As these conditions necessarily are not constant in all 
cases, we find, as might be expected, a corresponding 
variation in different plants by which they have accom- 
modated themselves to these varying conditions. 
Most of the lower green plants are aquatic and all 
their cells are equally exposed to the medium in which 
they live. These plants being unicellular or composed 
of simple cell-aggregates made up of similar cells, 
each cell is capable of performing the different plant 
functions, which in more highly specialized plants are 
relegated to special cells. Each cell of these simple 
plants absorbs water containing the necessary food 
constituents in solution, and as all the cells contain 
chlorophyll, all are able to decompose the carbon di- 
oxide dissolved in the imbibed water. The free oxygen 
needed by the plant is also taken in with the water. 
Associated with the aquatic habit of these plants is the 
power of active locomotion so often seen in their repro- 
ductive cells. 
The marine forms allied to these simple alge have 
become much changed in some respects, and notably in 
262 
