256 PLANT PHYSIOLOGY 



vival value in the formation of a desert flora. In fact, according to 

 Maximov (1923), the transpiration from many xerophytes is 

 commonly high and their real defensive mechanism lies not in 

 checking transpiration but in possessing cell sap of a high osmotic 

 concentration, thus enabling them to recover after wilting. 



Ecological Types on Basis of Water Supply.— Water is one of 

 the prime factors in determining the form and distribution of 

 plants. The development of organized life centers about water, 

 which is necessary in fairly large amounts for protoplasm to func- 

 tion at all. Plants growing in marshes, lakes, and places where 

 there is never any danger of a shortage in the water supply are 

 called hydrophytes. Those growing under conditions of a moderate 

 water supply are called mesophytes, and to this group belong not 

 only most crop plants but most of the land plants. 



Plants which grow in regions where water is very scarce are 

 called xerophytes. Of such plants there are many different types, 

 but among the better known are the succulents (cacti, Sedum, 

 Agave, and aloes), which have a small surface compared with their 

 volume. Water is conserved within the plant and is transpired 

 very slowly because of the thick, waxy cuticle and the water hold- 

 ing capacity of the mucilages and gums contained. But photo- 

 synthesis also is checked in such plants, and their growth is ex- 

 tremely slow. 



Other xerophytes are the thin-leaved forms such as the sage 

 brush, so familiar on the western deserts of the U. S. A. These 

 plants have a high transpiration, but they are thus able to live in 

 extremely hot localities, and their deep root systems permit them 

 to replenish the water lost. In extremely dry weather, however, 

 water may be still further conserved by the shedding of leaves and 

 branches. The hairy coverings on the leaves have been thought 

 to be a check on evaporation, but, as mentioned above, the effi- 

 ciency of these hairs has been found to be very low in this respect, 

 and Maximov considers that they function rather to protect the 

 chlorophyll against the bright light and to protect the plant against 

 grazing animals. These plants, unlike the succulents, have a 

 very high osmotic pressure in their cells and can thus draw most 

 of the water from the soil. There are still other types of xero- 

 phytes, but this is sufficient to indicate the variation within the 

 group. 



