THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



599 



LEAF 

 BLADE 



LIGULE 



bundle. In the lower part of the leaf 

 this has a collateral structure, with an 

 arc of thick-walled phloem on the outer 

 (abaxial) side, and a very reduced xylem 

 consisting of one strand of protoxylem 

 elements and a few small metaxylem 

 tracheids. These latter lie on both the 

 inner and outer sides of the protoxylem, 

 which is therefore mesarch. Further 

 up the leaf the xylem becomes exarch 

 and the protoxylem strand is replaced 

 by a cavity in which traces of ligniiied 

 walls remain. Often the metaxylem 

 tracheids are also resorbed in the same 

 way, only cavities marking their position. 

 The phloem has sieve tubes in some 

 species, but none have been observed in 

 Isoetes lacitstris. 



At each side of the bundle in the 

 expanded leaf base may be seen a narrow 

 cavity containing mucilage and partly 

 disorganized cells. This is of unknown 

 significance, but it is interesting to notice 

 that exactly similar cavities flank the leaf 

 trace in the fossil plant Lepidodendron, 

 where they are called parichnoi. 



The leaf base has two colourless wings, 

 which extend a considerable way round 

 the circumference of the stem. Between 

 them lies a cavity in the leaf tissue, the 

 fovea, opening to the ventral (adaxial) 

 side, and containing a sporangium * 

 (Fig. 6io). 



The opening of the fovea is partially 

 covered by a membrane growing down- 

 wards from above, called the velum. 

 Immediately above the fovea is the 

 heart-shaped ligule. This is much 

 larger and more complex than in 

 Selaginella, though it grows from a single 

 cell. It arises from a cavity in the leaf tissue, the ligular pit, and consists 

 of four portions, firstly a sheath of glandular-looking cells, in contact with 



VELUM 



SPORANGIUM 



Fig. 6io. — Isoetes lacustris. A single 

 sporophyll viewed from the ad- 

 axial side showing the ligule and 

 a sporangium in the sporangia! 

 cavity. {After Troll.) 



* A remarkable peculiarity, found only in the specimens from a single lake in the Vosges 

 Mountains, is the development of detachable vegetati\e buds, or gemmae from the inner 

 side of the leaf base, just below the position of the sporangia. The latter are completely 

 abortive and this particular variety relies solely on vegetative propagation, like certain 

 viviparous Flowering Plants, 



