THE VASCULAR CRYPTOGAMS 49 



enter the successive segments of the lamina, and by their 

 further ramifications supply its vascular system (see Fig. 

 1 8, B). As we trace the bundles into the finer veins of the 

 leaf, we find that the upper part of the phloem gradually 

 dies out, so that the ultimate branches of the bundle 

 system come to be collateral instead of concentric. This 

 is very generally the case in Ferns. 



If we now endeavour to sum up what we have learnt of 

 the vascular system of the Male Fern, we see that its most 

 striking peculiarity consists in the polystely of the stem, 

 where each strand of wood and bast resembles an entire 

 central cylinder rather than a single vascular bundle. 

 As we follow the leaf-traces outwards, however, we find 

 that the steles assume more and more the character of 

 simple vascular bundles, until in the lamina they have 

 the same collateral structure as in the leaves of flowering 

 plants. It is evident that no sharp line can be drawn 

 between stele and bundle. 



Returning to the lamina of the leaf, we find that its 

 structure is distinctly bifacial. The mesophyll towards 

 the upper surface consists of closely-packed squarish cells, 

 forming a kind of palisade-parenchyma, though the 

 palisade form is not well marked. The lower portion of 

 the mesophyll, on the other hand, is made up of 

 irregularly branched cells, attached to each other by only 

 small parts of their surface, so that large intercellular 

 spaces are left between them. This tissue is thus a 

 typical spongy parenchyma. All the cells of the meso- 

 phyll contain abundant chlorophyll granules (see 

 Fig. 28, A). 



The epidermis of the lower surface alone bears the 

 stomata, which are very numerous (see Fig. 24). 



The stomata are characteristic : each pair of guard- 

 4 



