598 



A TEXTBOOK OF THEORETICAL BOTANY 



Fig. 608. — Stigmaria ficoides. Transverse section of 

 root showing similarity of structure to the root of 

 Isoetes. Stigmaria is a palaeozoic fossil. 



Anatomy of the Leaf 



The leaf is from 2 to 6 in. long and comprises an expanded and thickened 

 base, with an upper portion which is awl-shaped and sharply pointed. Every 

 leaf seems to be potentially a sporophyll, as traces of abortive sporangia 

 occur at the base even of the vegetative leaves. A similar condition is found 

 in Lycopodium selago, which likewise has no specially organized strobili. 



The leaf has no apical cell and growth is intercalary, from near the base, 

 as in Monocotyledons. The upper part of the leaf shows, in transverse 

 section, four large intercellular cavities or lacunae separated by parenchy- 

 matous diaphragms (Fig. 609), a condition which is usually found in sub- 

 merged aquatic plants. In the centre runs a single, unbranched vascular 



Fig. 609. — Isoetes lacustris. Transverse section of 

 the leaf showing the central vascular strand and 

 large intercellular cavities. 



