72 THE BIOLOGY OF FLOWERING PLANTS 



letting the air escape. The amount of water taken up is very 

 considerable — according to Goebel's measurements (1893), 

 from 40 to 80 per cent, of the weight of the root. A distinc- 

 tion between absorption by a velatnen and by an earth root 

 is that in the former case the whole surface of the root is 

 active, and in the latter only the restricted root-hair zone. 

 The wet root appears green, the chlorophyll of the cortex 

 being visible through the wet velamen. The water passes 

 more slowly into the cortex and conducting tissues of the 

 roots. When the velamen once more dries up it acts as a 

 covering, protecting the root from drying out to a very 

 appreciable extent. The supply of salts must be rather 

 precarious, depending on the detritus washed down by rain 

 and collecting about the roots. Velamina seem to be 

 possessed by all epiphytic orchids, e.g. Epidejidron noctur- 

 mim, Vanda furva, Odontoglossum Barkeri. They are less 

 general among the Aroids, e.g. Anthuriiim egregium and 

 A. acaule. 



The velamen is not only common among epiphytic 

 orchids and aroids, it is also widespread among terrestrial 

 forms such as Sobralia and Phajus, though generally less 

 well developed. As there is no sharp line between ter- 

 restrial and epiphytic forms, this is not surprising. A 

 velamen has, however, been found in a number of mono- 

 cotyledons which are strictly terrestrial, by Goebel (1922). 

 These belong to the Liliaceae, e.g. Agapantlms iimhellatiis , 

 Aspidistra elatior, and Amaryllidaceae, e.g. Crinum lofigifoliuniy 

 Clivia nohiJis. The occurrence of a well-developed velamen 

 on the roots of these terrestrial South African plants, which 

 have certainly no epiphytic tendencies, is of great interest. 

 These roots produce also root hairs, and it is to be supposed 

 that the velamen functions chiefly in producing a rapid 

 increment in the water available for growth when the rains 

 set in, and perhaps a certain reserve in dry weather. 



The occurrence of the velamen in earth roots of orchids 

 and aroids might be taken to be a relic in species descended 

 from epiphytic ancestors ; that it is found in roots of typical 

 geophytes can only indicate that it is a structure which may 



