THE LEAF AND ITS FUNCTIONS 67 



plump and turgid with water they tend to pull apart, thus 

 enlarging the opening. On becoming limp and partially collapsed, 

 however, they spring together again and close it. The degree 

 of stomatal opening is thus continually fluctuating as the water 

 supply of the guard-cells rises and falls in response to changing 

 internal or external conditions. 



The mesophyll consists of tissue which is characteristically 

 thin-walled, soft, and green. The cytoplasm within its cells 

 contains very small, roundish bodies, denser than the rest of the 

 hving substance, and green in color. These are the chloroplasts 

 which contain within them the green pigment chlorophyll, to 

 which the characteristic color of foliage is due. The mesophyll 

 is not a homogeneous tissue but in typical leaves is divided into 

 two main regions. That part lying next to the upper side 

 of the leaf is composed of cells which are elongated at right 

 angles to the leaf surface, packed rather closely together, and 

 provided with a great abundance of chloroplasts (Fig. 36). 

 This region is known as the palisade layer and here is carried 

 on most actively the process of food-manufacture or photo- 

 synthesis. The lower region of the mesophyll consists of a mass 

 of cells which are so very irregular in shape that large air-spaces 

 occur between them, and a very loose, sponge-like tissue, the 

 spongy layer, is produced. These air-spaces communicate 

 directly with the outside air through the stomata. Chloro- 

 plasts are present in the spongy layer, but not abundantly. 

 Through the exposure to the air of a large area of cell-surface, 

 opportunity is provided in this portion of the mesophyll for those 

 gas-exchanges which are continually taking place between the 

 leaf and the atmosphere, such as the absorption and excretion of 

 both carbon dioxide and oxygen, in the processes of photosynthe- 

 sis and respiration, and the evaporation of water in the process 

 of transpiration. 



Running through the blade are the fibro-vascular bundles or 

 veins, the channels by which the leaf tissues are kept in communi- 

 cation with the rest of the plant. The main veins are stout, 

 often projecting somewhat below the lower surface of the blade. 

 These break up into smaller and smaller veins, and finall}^ into 

 minute veinlets which consist of only a few cells. Each vein is 

 surrounded by a bundle-sheath of heavy-walled cells, to which 

 most of its rigidity is due. Within this are two tissues: The 

 wood, consisting largely (as elsewhere in the plant body) of olon- 



