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 algse have 

 become much changed in some respects, and notably in 



262 



