198 THE BIOLOGY OF FLOWERING PLANTS 



origin, growing at Carmel, California, the latter on the 

 foreshore in arid but not saline conditions, the former in the 

 open forest. Their conclusion is that extreme reduction 

 of water in the cell leads to a conversion of the hexose 

 polysaccharides into pentosans, which have an enormous 

 water- absorbing capacity. Aridity would thus lead directly 

 to succulence by a change in the carbohydrate chemistry 

 of the cell, and would automatically lead to an increase in 

 water-retaining capacity. In the particular case of Castilleja 

 this change to succulence would be a direct result of the 

 reduction of the cell water below a certain point. This 

 investigation opens up a whole vista of fresh possibilities in 

 the investigation of succulence. It is quite possible that 

 salt succulence may find its explanation on some such lines. 



§ 31. Assimilating Roots 



In some epiphytic orchids extreme reduction of the 

 leaf is accompanied by the formation of a root system 

 with abundant chlorophyll. In Tceniophyllum Zollingeri, 

 a Javan species described by Goebel (1889), the roots 

 are flattened, leaf-like, and pressed to the stems of the 

 palms on which the epiphyte grows. The velamen is 

 confined to the under surface of the root. Similar roots are 

 formed in many species of Angraecum. The leaves are 

 present only as minute scales. In the American Aeranthus 

 funalis, the assimilating roots are cylindrical and hang 

 partly in the air. It may be noted that the aerial roots 

 of many orchids possessing well-developed leaves have a 

 certain amount of chlorophyll. 



Reference may here be made to the remarkable assimi- 

 lating roots of the Podostemaceae and Tristichaceae, two 

 families inhabiting torrential streams in the tropics. The 

 root system is, in some cases, thalloid, closely applied to 

 the water- worn rocks on which the plant grows, and is the 

 only vegetative organ developed (see Fig. 23 and pp. 

 Ill, 294). 



