302 PSILOPHYTES 



particularly noticeable when the observer can examine the whole tract 

 from above. Here are found many thorny shrubs, including Acacia 

 Farnesiana and A. tortuosa, Parkinsonia aculeata, and Randia aculeata ; 

 as well as Cactaceae (including Cereus, Opuntia, and Melocactus) and 

 species of Agave. There are not a few laticif erous plants, including Plumeria, 

 Rauwolfia, and Calotropis ; also many shrubs with upwardly directed 

 leaves, or, as in the case specially of acacias, with leaves that execute 

 movements according to the intensity of the light, or, finally, showing 

 other methods of protection against excessive transpiration. In these 

 tropical bushlands there are some lianes, also epiphytic Bromeliaceae 

 and Orchidaceae, although atmospheric aridity checks any luxuriance or 

 abundance of growth. After more prolonged periods of drought, according 

 to Borgesen, 1 the leaves hang flaccid and more or less wilted ; yet there 

 is no regular shedding of the foliage. 



Allied to this type of bushland is subalpine bushland on tropical 

 mountains. This is composed of gnarled little trees with strongly xero- 

 phytic foliage. Epiphytic Vascular plants are lacking, though lichens 

 often abound on the trees. 2 



1 Borgesen and Paulsen, 1900. 



2 Junghuhn, 1852-4 ; Schimper, 1898 ; Engler, 1891 ; Volkens, 1897 ; O. Stapf, 

 1894. 



