SUN LEAVES AND SHADE LEAVES 471 



leaves from the exterior and the interior of the crown of a de- 

 ciduous tree, or such an evergreen species as the live oak or the 

 olive, the sun leaves are usually found to be lighter-colored, of 

 smaller area, thicker, and of more xerophytic structure than the 

 shade leaves. The difference in size may be very great, the 

 smallest sun leaves sometimes not covering more than a tenth 

 of the area of the largest shade leaves on the same plant. There 

 is usually, also, a notable difference in the form of the two 

 kinds, the sun leaves being narrower in proportion to their 

 length. Sun leaves are often several times as thick as shade 

 leaves, and have far more completely developed palisade layers. 

 The latter may even (in leaves grown in dense shade) be quite 

 lacking, and the regular palisade cells be replaced by loosely 

 arranged, funnel-shaped cells, with their broader ends toward 

 the epidermis. Sun leaves have a much stronger fibro-vascular 

 framework than those developed in a comparatively feeble light. 

 In the case of plants which have the leaves more or less hairy 

 or scaly, the covering of these epidermal outgrowths is, as might 

 be expected, much more dense on the sun leaves. 



Probably the work of all kinds done by the sun leaves is far 

 greater than that done by shade leaves of the same species. 

 This is partly due to the much greater supply of energy daily 

 received by the former from the sun, and it is also due to their 

 more capacious conducting system and greater supply of chloro- 

 plasts. The transpiration of a given area of sun leaves is at 

 times tenfold that of the same area of shade leaves (both being 

 placed for the time in full sunlight). 



448. Transition of a plant from shade conditions to sun con- 

 ditions. It is characteristic of many kinds of forest trees that 

 the young seedlings are much more tolerant of dense shade than 

 the adult trees. Sometimes their seeds will hardly germinate 

 at all unless thoroughly shaded, and the young trees for the 

 first few years flourish best in the shade. Afterwards most trees 

 need a good deal of sunlight, but they may live long with a 

 scanty supply of light. The red spruce sometimes lingers on for 



