CRYPTOGAMS 441 



remains attached to the parent prothallium and absorbs nourishment 

 from it, until, by the development of its own roots and leaves, it is 

 able to nourish itself independently. The prothallium then usually 

 dies. The stem developed from the embryonic rudiment may be 

 either simple or bifurcated, erect or prostrate ; it branches without 

 reference to the leaves, which are arranged spirally or in whorls, 

 or occupy a dorsiventral position. Instead of rhizoids, true roots 

 are produced, as in the Phanerogams (cf. Fig. 167). The leaves also 

 correspond in structure with those of the Phanerogams. Stems, 

 leaves, and root are traversed by well-differentiated vascular bundles, 

 and the Pteridophytes are, in consequence, designated Vascular 

 Cryptogams. The bundles of the great majority of Pteridophytes are 

 constructed on the concentric type (cf. Fig. 401, and Fig. 125, p. 115). 

 Secondary growth in thickness, resulting from the activity of a special 

 cambium, occurs only occasionally in existing forms, but it was char- 

 acteristic of the stems of certain extinct groups of Pteridophytes. 



The SPORES are produced vegetatively in special receptacles termed 

 SPORANGIA, which occur on the asexual generation, either on the 

 leaves, or less frequently on the stems in the axils of the leaves. 

 The leaves which bear the sporangia are termed SPOROPHYLLS. The 

 sporangium consists of a wall enclosing the sporogenous tissue, the 

 cells of which, becoming rounded off and separated from each other 

 as spore -mother -cells, give rise each to four tetrahedral spores 

 (spore-tetrads). The cells of the innermost layer of the sporangial 

 wall are rich in protoplasm, and constitute the TAPETUM. This 

 layer persists in the Lycopodineae, but in the case of the Ferns and 

 Equisetineae the walls of the tapetal layer become dissolved. In 

 the course of the development of a sporangium the tapetal cells 

 then wander in between the spore-mother-cells, so that the spores 

 eventually lie embedded in a mucilaginous protoplasmic mass, the 

 PERIPLASM, from which they derive nourishment. The wall of the 

 mature sporangium is formed of one or a number of layers of cells. 

 The unicellular spores have walls composed of several coats. 



The spores of the majority of the Pteridophytes are of one 

 kind, and give rise on germination to a prothallium, which produces 

 both antheridia and archegonia. In certain cases, however, the 

 prothallia are dioecious. This separation of the sexes extends in 

 some groups even to the spores, which, as MACROSPORES, developed 

 in MACROSPORANGIA, give rise only to female prothallia ; or as MICRO- 

 SPORES, which are produced in MICROSPORANGIA, develop similarly 

 only male prothallia. In accordance with this difference in the spores, 

 a distinction may be made between the HOMOSPOROUS and HETERO- 

 SPOROUS forms of the same group ; but this distinction has no 

 systematic value in denning the different groups themselves, as it has 

 arisen in groups in other respects quite distinct. 



Compared with the Bryophyta, the asexual cormophytic generation 



