NATURE. OF PLANTS 37 



and moisture loving plants are not obliged to conserve the amount 

 of water received and these two forces, feeble light and moisture, 

 produce a larger and looser arrangement of tissues which is 

 favorable to an interchange of gases and transpiration (Fig. 

 27). Particularly is this noticeable in aquatics as in the water 

 lilies and many rushes, etc., where the tissue is loose and spongy 

 and permits a ready circulation of gases from the leaves to the 

 roots and to all parts of the plant body even when submerged. 

 This loose arrangement of tissues also renders water plants very 

 buoyant and consequently they are less liable to injury from the 

 currents of the water. Aquatic plants have little need of strength- 

 ening and conducting tissues because they are supported by the 

 water and the crude foods being absorbed by nearly all parts 

 of the plant body, do not require elaborate bundles for their 

 conduction. The palisade arrangement of tissues is also favor- 

 able to photosynthesis. In intense light the movements of the 

 protoplasm usually distribute the chloroplasts along the sides 

 of the palisade cells so that they are edgewise to the light, but 

 in feeble light they are distributed throughout the cell with their 

 broad surfaces to the light. Just why this disposition of the 

 plastids is made is not known but that it is of vital importance 

 to the formation and distribution of substances in the cell is not 

 to be questioned. We now understand why the shade leaves are 

 thin and broad and soft while leaves of desert plants and those 

 exposed to severe drying winds of summer or winter are thick, 

 compact, firmer and often leathery and hairy. 



One of the most interesting adaptive features of leaves is seen 

 in the leaf fall of our deciduous trees and shrubs. In the tropics 

 the leaves remain on the perennial plants often for long periods, 

 but in temperate climates the severe winters necessitate the 

 annual dropping of the leaves except in a comparatively few 

 evergreens where the thick leathery leaves are able to endure 

 such conditions. In our deciduous plants, when the conditions 

 are no longer favorable for the performance of leaf work, the 

 cells at the base of the petiole begin to divide and a delicate 

 layer of cells, the separating layer, is formed across the petiole 

 (Fig. 28). Various conditions induce this growth for there is 



