TISSUES AND SIMPLE ORGANS. 79 



A cross-section of the stem of a vascular cryptogam (Selaginella 

 incequalifolia) may also serve to show the structural relations of 

 the upright and semi-upright (hence more or less firm and elastic) 

 stems of the fern-leaves. 



Fig. 45 represents the peripheral mechanical ring of the 

 stem. In the centre lie three vascular bundles surrounded by a 

 loose connective tissue with large intercellular air-spaces. We can 

 now also understand the structure of a single vascular bundle of a 

 fern (see Fig. 46), which in many respects resembles that of Sela- 

 ginella. The leptome in the form of two crescents lies in contact 

 with the plate consisting of woody parenchymatous and tracheal 

 elements; the fascicular conducting-parenchyma s surrounds the 

 albumen-conducting elements in the form of a ring. The protect- 

 ive sheath is shown at <?, the strengthening layer at w. I wish to 

 state at this point that according to more recent investigators the 

 majority of vascular cryptogams do not possess a true vascular sys- 

 tem ; the tracheal elements, in most cases, prove to be tracheids. 



Furthermore, in the group of vascular cryptogams there is 

 represented a wholly different type of structure ; this type is well 

 illustrated in the Equisetacece. For special functions, and hence 

 explanatory from a physiological standpoint, the stem of Equise- 

 tum has an essentially different structure from the stem of Sela- 

 ginella and the leaf-stems of related ferns. On account of the 

 rudimentary leaf -development among the Equisetacece, assimilation 

 on the part of the leaf is almost zero ; only the toothed leaf-sheaths 

 are present, and these function at the same time as mechanical 

 structures. The stem with its branches must therefore perform 

 the function of assimilation. To suit this requirement the mechan- 

 ical and assimilating systems are 

 alternately arranged on the outer 

 surface. A glance at Fig. 47 will 

 explain this arrangement. Be- 

 tween the bast-ribs (black) lie the " 



. . , FIG. 47. Transverse section of the 



assimilating tissues gr ; inside of stem of Equisetum htemale. 

 the bast-ribs are found the conduct- (Diagramatic.) 



ing bundles Z, and between these the large intercellular air-pas- 

 sages. (This figure also illustrates the rare case of an inner and an 

 outer ' ' protective sheath, ' ' concerning which more particular men- 

 tion will be made later. The dotted lines indicate their courses.) 



