5i6 A TEXTBOOK OF THEORETICAL BOTANY 



fusions, and normally each lamina segment receives one vein. The reduced 

 character of the genus is shown by the presence of pseudo-veins, consisting 

 of sclerenchyma cells without any vascular tissue, which either take the place 

 of the regular veins or form direct prolongations of them. They are obviously 

 imperfect veins and can hardly be anything but reduced structures. 



There are naturally no stomata in a single-layered lamina, and the cuticle 

 of the cells is extremely thin. On the other hand, the side walls are thick 

 and strongly pitted. Contrary to the behaviour of other plants, Hymeno- 

 phylhim can absorb water through its leaves. The lamina cells are very 

 permeable, and a drop of water placed on the leaf surface does not readily 

 run off but spreads and sinks into the cells. This capacity must be of great 

 importance to the smaller rootless species, whose vascular strands are so 

 much reduced that it would be virtually impossible for the water supply of 



; n 



^d 



Fig. 512. — Hymenophylhon timbridgeuse. Transverse 

 section of the lamina of a pinna showing midrib and 

 thickened margin. 



the whole plant to pass along them. The saturated air and heavy rainfall of 

 the tropical rain forest seem indeed to have induced in HymenophyUum 

 characters which are those of a semi-aquatic plant. 



The leaf margin is often formed of sclerotic cells, recalling the leaf margin 

 in some flosses like Mniiim. This serves the double purpose of preventing 

 tearing of the lamina and of keeping it taut, like the skin of a drum, thus 

 preventing the flaccid drooping which would be inevitable in so thin a structure 

 if it were not mechanically stiffened. The leaf veins contain a very small 

 number of vascular elements, but each is surrounded by a sclerotic sheath 

 which is an important reinforcement of the lamina. 



The growth of the young lamina is quite different from that of other Lepto- 

 sporangiatae. The margin consists of a continuous row of narrow^ cells, 

 shaped like half-discs, arranged side by side. These cells only divide parallel 

 to their inner, flat faces, thus building up the single layer of cells. In the 

 lowest part of the leaf the segmentation of the marginal cells by alternating 

 oblique walls, which is characteristic of other Ferns, may occasionally be 

 seen, but in the members of the Hymenophyllaceae which have thicker leaves, 

 e.g., Trichomanes renifortne, the leaf is still built up from a margin of half- 

 disc shaped cells, the increased thickness being acquired by subsequent 



