6o6 



A TEXTBOOK OF THEORETICAL BOTANY 



latter breaks out from the prothallus. Up to this stage there has been no 

 stem apex, but a scale called the cotyledonary sheath makes its appearance 

 at the base of the cotyledon, and between them is formed the meristematic 

 group which becomes the apical tissue of the stem. The stem is, however, 

 for long very ill-defined, and the apex is only recognizable as such because 

 it is from this region that all the leaves after the cotyledon are produced. 



The foot increases in size by absorption of all the remainder of the 

 prothallus until it fills the spore cavity. The cotyledon and the first root 

 grow out, at first horizontally, and the prothallial tissue, which expands 



ROOT 



PROTHALLUS 



COTYLEDONARY 

 SHEATH 



STEM APEX 

 LIGULE 



COTYLEDON 



PERISPORE 



Fig. 6 1 6. — Isoetes lacustris. Section through megaspore and female prothallus 

 showing developing embryo and disorganising prothallial tissue. {After Liebig.) 



with their growth, encloses each in a sheath, which is eventually ruptured. 

 The cotyledon, which resembles anatomically the other leaves, then turns 

 upwards and the root downwards (Fig. 6i6). 



Relationships 



The afiinities of the other Vascular Crytogams which we have discussed 

 are in most cases fairly clear, but the unique characteristics of Isoetes make 

 it a more diflicult problem. Except for the Filicinean antherozoid, however, 

 the bulk of the characters, such as the simple leaf with its one leaf bundle 

 and ligule, the sporangia borne on the adaxial leaf surface, the spores, and 

 especially the gametophytes and the embryology strongly suggests a relation- 

 ship to the Lycopodiales. 



The presence of a parichnos in the leaf base and the anatomy of the root 



