6o8 A TEXTBOOK OF THEORETICAL BOTANY 



Sub-tropics. Though somewhat different in their habit they are sufficiently 

 related to form only one family, the Psilotaceae. The shoots of Tmesipteris 

 are unbranched and leafy (Fig. 617), while those oi Psilotum branch dichoto- 



FlG. 617. — Tmesipteris tannensis. Plant 

 growing epiphytically on the stem of a 

 Tree Fern. 



mously and have only scale-leaves. We shall describe as a type Psilotum 

 triqiietrum. 



Psilotum triquetrum 



This is the principal species, the only other one, P. complanatum, which 

 is much rarer, differing chiefly in its flattened stem. 



The plant is slender and shrubby, rarely as much as a metre in height, 

 with green, ridged, dichotomous stems (Fig. 618). Superficially it is not 

 unlike a leafless plant of the Whortleberry, Vaccinium myrtillus. It grows in 

 humus pockets, usually on the stems of Palms or Tree Ferns, but also in rock 

 clefts, and often hangs downwards. The stems are perennial and somewhat 

 xerophytic in structure. 



At its base the stem merges into a slender, dichotomous rhizome, which 

 is clothed with brown rhizoids, except at the apices, which are naked and 

 white. There are no roots and the rhizome forms no leaves. The rhizome 

 lives for several years underground, forking frequently and irregularly until 



