THE PLANT AND ITS ENVIRONMENT 



161 



Toward light, as toward temperature, plants display certain 

 inherited adaptations. Some green plants are able to thrive in 

 light of comparatively low intensity and are said to be tolerant 

 (Fig. 79), in the sense that they will tolerate a considerable degree 

 of shade. Others will grow normally only where the illumination 



Fig. 79. — Plants tolerant of shade. These broad-leaved plants of Viburnum 

 are able to thrive on the forest floor, where the lower branches of the forest trees 

 have died from lack of light. 



is good, and are said to be intolerant (of shade). Some are sensi- 

 tive to intense light and thus cannot live in the open, whereas 

 others, either through the greater resisting power of their proto- 

 plasm or through various structural modifications, are able to 

 withstand the most brilliant sunlight. 



Gravity .^ — Gravity in its effect upon plants is unlike tempera- 

 ture and light in that it is both continuous in action and constant 

 in amount. It is a very important factor in determining the 

 direction of growth in plant parts and, like light, it affects 

 different organs in different ways. Any reaction to the stimulus 



