202 



PTERIDOPHYTA. 



Mosses, having an epidermis, a ground tissue with variously dif- 

 ferentiated cells, and a highly developed vascular system. The 

 vascular bundles, like those in the Monocotyledons, are without 

 cambium, and closed ; they are therefore incapable of any increase 

 in thickness. In general the bundles are concentric, with the 

 bast round the wood (Fig. 203). The wood is almost entirely 

 made up of scalariform tracheides. 



In Iso'ftes a secondary thickening takes place by a cambium, which is formed 

 inside the cortex, constructing secondary cortex to the exterior, and secondary 

 wood towards the interior. Botrychium has also a thickening growth. Collateral 

 vascular bundles occur in Osmundacecc, Equisetacece, and the leaves of many 

 Pulypodiacece, etc. 



It is a point of special interest, that the gigantic forms of Ferns, 



Equisetums, and Club- 

 Mosses (which flourished 

 in earlier geological 

 periods, when these classes 

 attained their highest de- 

 velopment) possessed 

 some means of increasing 

 in thickness. 



The sporangia are in 

 all cases capsule-like, and 

 burst open when ripe to 

 eject the spores. They 

 are nearly always situated 

 on the leaves (in Li/copo- 

 diacece, in the axils of the 

 leaves, or above these, on 

 the stems themselves). In 

 some forms (LEPTOSPOR- 

 ANGIATJE), the sporangia 

 are developed from a single epidermal cell ; in others (EusPOR- 

 ANGIATVE), from a group of epidermal cells, or from cells which 

 lie beneath the epidermis. In the first group a primitive mother- 

 cell (archesporium) is formed, which divides commonly into sixteen 

 special mother-cells. In the latter group, on the other hand, a 

 number of primitive spore-mother-cells are developed. In each 

 sporangium three different tissues are generally developed ; an 

 innermost sporogenous one (s in Fig. 20 i A), which arises from 

 the archesporangium ; an outermost one, which forms the ivall (a), 



FIG. 203. Portion of the stem of a Fern. Above 

 is seen the transverse section, with vascular bundles 

 of different form and size. The rhombic figures on 

 the side of the stem are leaf -scars. 



