190 APPLIED BIOLOGY 



growing in window-boxes ; but many grasses and other plants 

 have leaves nearly vertical or erect, and others hold their leaves 

 edgewise. In such cases light does not strike the surface 

 directly. Leaves so arranged are probably very sensitive 

 and the indirect light is better suited to their work. 



Climbing plants grow so as to arrange their leaves with ref- 

 erence to light as definitely as do plants with erect stems. 

 Many vines spread their leaves over the outermost branches 

 of trees, especially in dense forests. In fact, the climbing 

 habit is believed to have its purpose in securing the best pos- 

 sible exposure of leaves to light. 



It is interesting to note that even plants adapted to grow- 

 ing in the shade of forests appear to have leaves arranged for 

 best exposure to light. This shows that while such plants 

 are adjusted to shade, they must have as much of the diffuse 

 light as possible under the conditions. 



The leaves of some plants attain the best exposure by move- 

 ments, such as have been mentioned, adjusting their leaves 

 to the amount of light best suited to them. 



Finally, the forms of leaves appear to be of some meaning 

 in connection with best exposure to light. The triangular 

 leaves of some plants seem to fit together better than rounded 

 leaves would on the same arrangement of branches. The 

 numerous kinds of notches, lobes, and branching of leaves 

 allow light to pass through to lower leaves. If one examines 

 plants with such leaves exposed to bright sunlight, it is evi- 

 dent that some light passes through the spaces between leaves. 



There are numerous other ways in which certain plants 

 have their leaves arranged in order to get the best possible 

 light. The best for a given plant does not mean the most 

 light, for at times sunlight may be so intense as to be injurious 

 to some plants. However, this is usually provided for by 

 movement in some species and by thick cells which reduce 

 the intensity of light that reaches the more sensitive middle 

 tissues of leaves. 



