CHAPTER IV 



THE ECOLOGY OF LEAVES 



The principal function of leaves is food manufacture. ]Many 

 leaves are modified to perform other special functions and in some 

 plants stems are more important than leaves as food manufacturing 

 organs. In some cases, furthermore, leaves are reduced to mere 

 scales and perform no function at all. As a general rule the greatest 

 danger to which leaves are subjected is excessive transpiration and 

 for this reason the structure of the leaves is in a sense a compromise 

 between what would be best as a photosynthetic organ and what is 

 necessary to safeguard against too rapid a loss of water. 



24. Structure and Arrangement of Tissues.— The tissues of the 

 leaf that vary most with variations in the environment are those 

 that contain chlorophyll. Variations in the fibrovascular bundles 

 and their sheaths, so far as our present knowledge of them goes, do 

 not seem to be of much ecological significance. 



The majority of plants that grow in exposed places where there is 

 an adequate supply of water have leaves with no chlorophyll in the 

 epidermis except in the guard cells of the stomata, but with chloro- 

 phyll in all other cells except those of the fibro-vascular bundles and 

 their sheaths. The tissues that contain chlorophyll are called col- 

 lectively, chlorenchyma. In the typical leaf of which we are speak- 

 ing the upper half of the chlorenchyma is made up of elongated cells 

 with their long axes perpendicular to the epidermis. These cells 

 because of their shape are called palisade cells. The lower half of 

 the chlorenchyma, on the other hand, is made up of irregularly- 

 shaped parenchyma cells and has numerous intercellular spaces. 

 For this reason it is called spongy parenchyma (Fig. 15). 



If now we examine the leaves of plants growing in progressively 

 wetter places we shall find a progressive decrease in the amount of 

 palisade tissue and a progressive increase in spongy parenchyma until 

 in the case of leaves which are submerged in water there is no palisade 

 tissue at all, the chlorenchyma being made up entirely of spongy 

 parenchyma with very large intercellular spaces (Fig. 16). The 

 leaves of plants that grow in wet places, and especially in dense 

 shade, often have chlorophyll in the epidermal cells and for this 

 reason are dark green in color. 



(41) 



