CH. Vl] ARBOREAL HABIT. 89 



its fellows, gains access to the light, and in shading 

 lower trees tends to starve a possible rival, and thus 

 better its own chance of keeping possession of the light. 

 At the same time in keeping back the aerial growth of 

 its rivals it starves their roots and thus keeps its own 

 roots free from undue competition. Many facts go to 

 prove that this struggle for light is an important feature 

 in the environment of plants. From this point of view it 

 is possible to understand the advantage of the climbing 

 habit in a plant, for it is thus enabled to reach the light 

 by a small expense of actual stem-production : it succeeds 

 by adaptation, instead of by the patient construction of a 

 column-like trunk of massive strength. The same thing 

 is true of epiphytes, i.e. plants which perch and root on 

 others, such as the innumerable orchids, ferns, Bromelias 

 &c. of tropical forests, which do not necessarily exhibit great 

 extension of growth, but possess adaptations for securing 

 themselves and for obtaining food in their aerial position. 



Granted that trees grow up into the air in a com- 

 petitive search for light, how are they guided, and how 

 enabled to carry on the search ? The fact that plants 

 grow straight up, even when forced to germinate in the 

 dark, proves that there exists a directive tendency, in 

 which light plays no part. And when it is found that 

 all over the world the trees grow vertically, it is im- 

 possible to help suspecting that the force of gravity, 

 which all over the world acts in the direction of the 

 earth's radius, is the guiding influence. 



This is the fact ; just as the root of a bean grows 

 vertically down, so the plumule grows vertically up. Both 



