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 alge, 
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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