[Chap. VIII THE TISSUE SYSTEM OF LEAVES 75 



Here it need be emphasized only that external factors — light, water, and 

 a warm temperature — are the most important prerequisites for their 

 opening. 



Summary, A leaf is a more or less flattened organ that develops from 

 a leaf primordium, which originates in the meristematic tissue at the 

 growing point of a stem. The blade may be sessile, or it may be sup- 

 ported at some distance from the stem by a petiole. The latter is essen- 

 tially a structure containing vascular bundles surrounded by other 

 tissues. The blade is a flattened body covered by an epidermis which is 

 perforated by stomates that are opened and closed by guard cells. 

 Within the blade, the mesophyll consists of several layers of parenchyma 

 cells which are variously arranged and partlv in contact with the inter- 

 cellular air spaces that ramify throughout the leaf. The air spaces are 

 continuous with the outside atmosphere when the stomates are open. 

 Within and throughout the mesophyll there extends an ever-branching 

 vascular system of water-conducting and food-conducting tissues that 

 are in contact with most of the mesophvll cells, and never more than a 

 few cells removed. Some or all of the mesophvll cells contain chloroplasts 

 and chlorophyll. The larger veins of the leax^es are often ensheathed 

 with hard sclerenchvma fibers. These fibers increase the rigidity of the 

 blade; but as shown bv wilted leaves, they are not sufficient to hold the 

 blade upright. Owing to the formation of layers of cutin, external cell 

 walls of the epidermis may be more or less impervious to water. 



The mesophyll is a mechanism of living cells connected with the stem 

 by water- and food-conducting tissues, distributed in a labyrinth of air 

 passages through which water \'apor, oxygen, and carbon dioxide can 

 move freely. The whole is enclosed bv a cutinized epidennis, in which 

 are stomates that may be open in warm sunlight, thus connecting the 

 internal with the external atmosphere. 



