CHAPTER II 



THE CONDITIONS OF PLANT LIFE 



No matter how simple or how complicated they may 

 be, all plants agree in their essential life-processes, and 

 certain conditions are necessary for these. All feed, 

 grow, and reproduce, and all exhibit to a greater or 

 less degree the power of movement, although this is, 

 as a rule, much less evident than in animals. For the 

 manifestation of the various functions, certain external 

 conditions are essential. Thus a certain amount of 

 moisture is necessary in order that they may grow, 

 and of course the requisite food elements must be sup- 

 plied. In green plants, where alone, as we have seen, 

 the assimilation of carbon dioxide goes on, this is 

 dependent upon the presence of light, and there are 

 certain limits of temperature also which regulate the 

 activity of the plant. The amount of moisture, heat, 

 and light necessary may vary greatly, however, in 

 different plants. Many water plants, especially algse, 

 often flourish in water whose temperature is very near 

 the freezing point, and some of them may be actually 

 frozen into the solid ice without injury ; but these 

 same plants are quickly killed if they are placed, even 

 for a short time, in warm water. In strong contrast to 

 these are certain low plants, e.g. species of Oscillaria 

 and various bacteria, which thrive in hot springs im- 

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