THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



609 



it forms a coralloid mass (Fig. 619). All this time it is saprophytic and its 

 cortex shelters a mycorrhizal Fungus. At length it sends out almost naked 

 branches which turn upwards to the light and grow into the first green shoots, 



Fig. 618. — Psilotum triqiietnwi. Plants growing epiphytically 

 on the trunk of a Palm. 



which, although initially weak and small, get larger with successive years 

 until at length the fruiting stage is reached. 



These aerial shoots, which at the base are round and brown, like the 

 rhizome, become green and three-angled in their upper parts. Both rhizome 

 and green shoot grow by means of three-sided apical cells and that of the 

 shoot produces minute, bifid, scale-leaves, placed at first in three vertical 

 rows, an arrangement which is lost in the older parts. 



The upper leaves bear, apparently in their axils, short pedicels, on each 

 of which is a triad of closely united sporangia, which are homosporous and 

 measure 2 to 3 mm. across. The two cusps of the bifid leaf clasp the base 

 of the sporangium closely (Fig. 620). 



The morphology of this structure is obscure and has been variously 

 explained. The opinions held may be reduced to two. Firstly, that the whole 



