596 FACTORS AFFECTING GROWTH 



to a destruction of enzymes or to changes in the physical state of the lipids. 

 The preponderance of the evidence at present available, hov^^ever, seems to 

 favor the coagulation of protoplasm theory as an explanation of the mechanism 

 of heat injury to protoplasm. 



III. Heat Resistance. — Certain tj'pes of tissues are more resistant to heat 

 injury than others. Tissues low in water content generally can endure rela- 

 tively high temperatures better than those of which the contrary is true. Dry 

 seeds and spores of some species have endured exposure to temperatures of 

 125° C. and even higher without loss of germinative capacity. 



In some plants otherwise susceptible tissues are protected against heat in- 

 jury because they are enclosed within tissues which have a low thermal con- 

 ductivity. In a forest which has been swept by a ground fire, to continue an 

 earlier example, it is usually noticeable that some of the trees have escaped 

 injury while others have been killed. Of individuals of the same species older 

 trees are more likely to survive than the 3^ounger ones. In mixed stands pines 

 are more likely to escape injury than hardwoods. These differences in sus- 

 ceptibility to injury from ground fires are apparently correlated with the thick- 

 ness of the bark layer (largely cork) which acts as an insulation between the 

 inner living tissues and high external temperatures. The bark is thicker on 

 old trees than on younger trees of the same species. Similarly the bark of 

 pines is usually thicker than that of most hardwoods. 



Effect of Radiant Energy on Growth. — The spectrum of radiant energy 

 (Fig. 79) ranges from the very long electric waves to the infinitesimally 

 short cosmic waves. All kinds of radiant energy, including light, vary in 

 several different ways, the most important of which are: (i) intensity, (2) 

 quality and (3) duration (Chap. XIX). 



Light is absolutely essential to all green plants because of its primary role 

 in photosynthesis. Numerous other effects of light upon physiological condi- 

 tions and processes in plants have been discussed in previous chapters. Among 

 these are: (i) chlorophyll synthesis, (2) stomatal action, (3) anthocyanin 

 formation, (4) temperature of aerial organs, (5) absorption of ions, (6) per- 

 meability, (7) rate of transpiration, and (8) protoplasmic streaming. Several 

 of the most important effects of light upon plants remain to be discussed in this 

 and the following chapters. 



In the present chapter we will deal with a few of the more striking 

 examples of effects of differences in the intensity, quality, and duration of 

 light and other forms of radiant energy upon the morphogenic development 

 of plants. 



I. Intensity. — Variations in the intensity of sunlight or artificial sources 

 of mixed radiations are almost invariably accompanied by at least minor varia- 



