52 THE ECOLOGY OF LEAVES 



transpiration, is a very influential factor, though probably it is not 

 the only one. 



The influence of water on leaf form is brought out even more 

 strikingly in the case of many water plants. In some of these, such 

 as the mermaid weed {Proserpinaca palustru) and lake cress {Radic- 

 ula aquatica), the aerial leaves, which are subjected to transpiration, 

 are nearly or quite entire, while the leaves that are in the water are 

 very much lobed or finely dissected. The water fern {Salvinia 

 natans) has broad floating leaves that do the work of photosynthesis 

 and very much dissected leaves that hang down into the water like 

 roots and serve as absorbing organs. In these cases there can be no 

 question of the influence of water on the form of the leaves. 



32. Leaves as Storage Organs.— Food storage in leaves is usually 

 only temporary. Permanent storage of food in these organs would 

 seriously interfere with their photosynthetic function. Food usually 

 accumulates in the leaves while it is being manufactured during the 

 day because it is synthesized more rapidly than it can be transported 

 to other parts of the plant. The transportation continues during 

 the night, however, so that by morning the leaf is ordinarily without 

 stored food. 



Nevertheless, there are some plants whose leaves are the chief 

 organs for permanent food storage. The century plant, which is a 

 species of Agave, is a well-known example of such a plant. (Fig. 19) 

 This plant grows vegetatively for from six to fifteen or twenty years, 

 storing up food and water in its large, succulent leaves. It then sends 

 up a flowering shoot which grows very rapidly and upon which a 

 large number of flowers and fruits develop and mature. By the time 

 this has been accomplished, the leaves have been drained of their 

 stored materials and the plant dies. 



Water accumulation in leaves, however, is often of considerable 

 importance. This is especially true of plants with fleshy or succulent 

 leaves (Fig, 19). Such plants are characteristic of some desert 

 regions and of salt marshes and other saline areas where, because of 

 the high concentration of the soil solution, absorption of water is 

 difficult. The retention of water by succulent leaves, which in some 

 cases is remarkably efficient, is accomplished by one or the other of 

 two general methods. The first of these is by strong cutiiiization, 

 layers of wax, or other modifications of the epidermis. The other 

 method consists in maintaining a very high concentration of the cell 

 sap, which effectively reduces the evaporation rate. Some succulent 



