404 PROTOPLASM 



sun should throw off a large part of itself or that Mars should 

 suddenly change its orbit. 



There are a number of types of secondary radiations due to 

 the primary types; thus, when ultraviolet light and X rays fall 

 upon a metal, they eject negative electrons. This emission of 

 electrons under the influence of light is called the photoelectric 

 effect. (It has been mentioned that the phenomenon is difficult 

 to account for on the wave theory of light but is readily inter- 

 preted in terms of the corpuscular theory.) 



IN THE LIVING WORLD 



Having acquired some knowledge of electromagnetic and 

 atomic radiation, we may consider the effect that these forms of 

 radiation have upon living matter. There is both experimental 

 evidence and the evidence that comes from daily experience to 

 show that organisms are at times very sensitive and at other 

 times very resistant to their radiation environment. Man's 

 knowledge of his radiation environment came first through his 

 familiarity with the influence of the sun's rays. Solar radia- 

 tion is intermediate in position in the spectrum; it is quite likely, 

 therefore, that those parts of the electromagnetic spectrum 

 beyond visible light will also have an effect on living matter. 



Visible Light. — The influence of visible light on life is so great 

 that we can select only a few of the less well-known examples for 

 consideration. Solar radiation warms the earth and the organ- 

 isms on it. It is the energy by means of which plants carry 

 on that primary chemical reaction upon which nearly all forms 

 of life depend, viz., photosynthesis. Plant stems and leaves 

 usually turn toward the light ; plant roots and many lower forms 

 of animals turn from it. Light determines the rate of growth 

 of plants. Dandelions grow to the height of the surrounding 

 grass; if the grass is cut, the flowers are formed on shorter and 

 shorter stems. The shape of trees is often determined by the 

 stopping of the growth of the limbs by light. The very sym- 

 metrical form of some fir trees is due to the influence of light. 



The growth rhythm, so typical of organisms in general, might 

 very hkely, in the case of plants, be attributable to temperature; 

 thus, seeds sprout and buds open in the spring because of warmth, 

 while plants die and leaves fall in the autumn because of frost. 

 Temperature is undoubtedly a factor. But Garner and Allard 



