Adaptation to Environment. 



309 



sunlight the leaves attained a size of 152 millimeters in 

 length and 20 millimeters' breadth (3). 



Sempervivum tectorum, which grows naturally in sunny 

 situations, and bears fleshy leaves 

 in rosettes near the ground, is 

 considerably modified when 

 grown in a light about one-sixth 

 as strong as direct sunlight. 

 Under this condition the inter- 

 nodes elongate so that the ro- 

 sette grouping of the leaves is 

 lost, the leaves formed are 

 smaller, and the amount of chlo- 

 rophyll is less. 



We can see that some of the 

 changes in habit here recorded 

 are directly adapted to the vari- 

 ations in the intensity of illumi- 

 nation. In a very dim light it 

 is advantageous to the plant to 

 lengthen the internodes at the 

 expense of all other parts, so that 

 the leaves may more surely and 

 quickly be brought into places of 

 greater illumination. Another 

 directly adaptive effect due to variations in light intensity 

 is the production of chloroplasts nearer the surface in 

 dimly lighted situations. This is well illustrated by our 

 wild Smilaxes, which grow in the shade of woods, and have 

 chloroplasts in the cells of the epidermis where they can 

 absorb the dim light to best advantage. 



193. Effect of Intense Illumination. Plants may adapt 

 themselves to varying degrees of illumination in still 



FIG. 161. 



% full sunlight ; 3, grown in % full 



