VEGETATION TYPES. 489 



tion has been described (paragraph 84). In many xerophytic 

 plants, especially in arid regions, the stomata are sunk in deep 

 cavities in the leaf so that the loss of water from them is not 

 so rapid. In many cases several of the above types of means 

 for retarding the loss of water from leaves are combined. 



4. Total absence of foliage leaves. This is a striking pecu- 

 liarity of many desert plants, or plants of very arid regions, 

 for example the cacti. Even in regions where mesophytes and 

 hydrophytes grow there are examples of plants devoid of leaves, 

 as seen in the horsetails and in many of the rushes. 



5. Thorns and spines. With the reduction of leaves there 

 often occurs a development of thorns and spines, especially in 

 dry situations. These often accompany reduced leaves on the 

 same plant, while in the cacti the spines often take the position of 

 leaves. 



6. Water reservoirs. In some plants the power of holding 

 considerable amounts of water in their tissues is very marked. 

 Such parts of the plant are real reservoirs for water storage. 

 It is a marked feature of the plants called succulents. In the 

 thick stems and leaves of the purslane (Portulaca) the middle 

 portion is largely devoted to the storage of water. The same is 

 true in the thick leaves of the stonecrops (Sedum). In some 

 begonias a layer of cells just underneath the epidermis of the 

 leaves serves as a water reservoir. Such plants when growing 

 under moderate climatic conditions have little need of water 

 storage. But when they inhabit alkaline or saline soils, or soils 

 in dry regions as they sometimes do, this habit stands them in 

 good stead. One can easily demonstrate their power of retaining 

 water in dry weather by pulling up the plants and leaving them 

 on the ground or by hanging them on the fence. They remain 

 fresh for a long time, while the ordinary plants quickly wilt. 

 It is readily seen from this how they are enabled to retain life in 

 dry situations during long periods, since they dole out the water 

 in small quantities. The most remarkable examples of the power 

 of plants to retain water are found in the cacti, where immense 

 trunks with no leaves, and a limited surface exposed to the air 



