158 BOTANY: PRINCIPLES AND PROBLEMS 



thrive under it by lowering the temperature gradually. This 

 resistance of plant tissues to heat and cold is also dependent to 

 some degree on maturity, for young and growing tissues are 

 much more susceptible to injury therefrom than older ones. In 

 general, within any particular species, the ability to withstand 

 high and low temperatures is correlated with the amount of water 

 in the cells and particularly in the protoplasm itself. Where 

 water is abundant, resistance is low; where it is scarce, resis- 

 tance is higher. 



There are well-marked inherited differences in the temperature 

 relations of plants. The optimum for an alpine plant must 

 obviously be far lower than for a native of the tropics. Certain 

 algae have their normal habitat in the water of thermal springs 

 and others in the frigid arctic seas. Melons have a much higher 

 optimum than peas, and some varieties of apple, peach, and plum 

 are distinctly "hardier" than other varieties. 



Light. — We have already noted the essential part which light 

 plays in the life of green plants through its influence upon the 

 process of photosynthesis. From light rather than from heat, 

 electricity or other sources, the plant derives the kinetic energy 

 which it stores up in potential form in its food; and in this 

 important capacity of an energy-provider light is therefore essen- 

 tial to all plants. This influence is evidently an indirect one, 

 however, particularly in the case of non-green plants, for most of 

 these thrive in the absence of hght as long as their food supply 

 holds out. 



Quite apart from its indirect role in nutrition, light exerts 

 certain direct effects. Most notable of these are the growth 

 movements made by plant parts in response to the stimulus of 

 illumination. Not all plants and not all parts of plants respond 

 in the same way to this stimulus. As a general rule the stem will 

 turn toward the source of light, the root away from it, and the 

 leaf, by the twisting of its petiole, will assume a position in which 

 the broad surface of the blade is at right angles to the light rays 

 (Fig. 77). Any movement which is a specific reaction to the 

 stimulus of light is known as a phototropism. A normal stem is 

 therefore said to be positively phototropic, a root negatively 

 phototropic, and a leaf neutrally phototropic or diaphototropic. 

 Although the results of phototropism are usually much more 

 noticeable where a plant receives its illumination from one side 

 only, it plays an important part in the orientation of plant 



