STUDIES IN CRYPTOGAMS 



203 



Other species of equisetum have only one kind of shoot a tall, 

 hard, leafless, green shoot with the spike at its summit. Equise- 

 tum stems are full of silex, and they are sometimes used for scour- 

 ing floors and utensils ; hence the common name " scouring rush." 



ISOETES (Pteridophyta) 



Isoetes or quillwort is usually found in water or damp soil on 

 the edges of ponds and lakes. The general habit of the plant is 

 seen in Fig. 300, a. It consists 

 of a short, perennial stem bear- 

 ing numerous erect, quill-like 

 leaves with broad sheathing bases. 

 The plants are commonly mis- 

 taken for young grasses. 



Isoetes bears two kinds of 

 spores, large roughened ones, 

 the macrospores, and small ones 

 or microspores. Both kinds are 

 formed in sporangia borne in an 

 excavation in the expanded base 

 of the leaf. The macrospores are 

 formed on the outer and the 

 microspores on the inner leaves. 

 A sporangium in the base of a 

 leaf is shown at b. It is partially 

 covered by a thin membrane, 

 the velum. The minute triangu- 

 lar appendage at the upper end 

 of the sporangium is called the 

 ligule. 



The spores are liberated by 

 the decay of the sporangia. They 

 form rudimentary prothallia of two 

 kinds. The microspores produce 

 prothallia with antheridia, while 

 the macrospores produce pro- 

 thallia with archegonia. Ferti- 

 lization takes place as in the mosses or liverworts, and the fertilized 

 egg-cell, by continued growth, gives rise again to the isoetes plant. 



FIG. 300. IsofcTES, showing habit 

 of plant at a ; b, base of leaf, show- 

 ing sporangium, velum, and ligule. 



CLUB-MOSSES (Pteridophyta) 



The club-mosses are low trailing plants of moss-like looks and 

 habit, although more closely allied to ferns than to true mosses. 

 Except one genus in Florida, all our club mosses belong to the 



