VASCULAR CRYPTOGAMS. 385 



degree ; hence, as the amount of moisture in the air varies they bend inwards or out- 

 wards, or sometimes in a spiral whorl, as in Barbula, 



The genus Polytrichum, to which the largest and most highly developed Mosses 

 belong, differs from the other genera in several points in the structure of its theca. 

 The teeth of the peristome are composed not simply of single pieces of membrane, but 

 of bundles of thickened prosenchymatous cells ; these bundles are horseshoe-shaped ; 

 the branches of two adjoining bundles directed upwards form together one of the 32-64 

 teeth. A layer of cells uniting the points of the teeth (Fig. 273, ep) remains, after the 

 casting off of the operculum and the drying up of the adjoining cells, as an epiphragm 

 stretched across the theca. The spore-sac is, in some species {e.g. P.piliferum), separated 

 from the columella by an air-cavity, which is penetrated, like the outer air-cavity, by 

 conferva-like rows of cells. In most species the seta is swollen beneath the theca, 

 forming the Apophysis, a phenomenon which is repeated in a somewhat different manner 

 in the genus Splachnum, where this part is sometimes expanded transversely as a flat 

 disc. 



GROUP III. 



VASCULAR CRYPTOGAMS. 



Under this term are included in one group the Ferns, Equisetaceae, Ophio- 

 glosseae, Rhizocarpeae, Lycopodiaceae, Selaginelleae, and Isoetese. As in the 

 Muscineae, the life-history of the plant is divided into two generations which are 

 extremely different both morphologically and physiologically. From the spore 

 proceeds first of all a sexual generation; from its fertilised archegonium is pro- 

 duced in the second place a new plant, which does not form sexual organs, but in 

 *their place a number of spores. In the Ferns, Equisetaceae, and Lycopodiaceae these 

 spores are all aUke ; the Rhizocarpeae, Selaginelleae, and Isoeteae, on the contrary, 

 produce two kinds of spores, large and small, Macrospores and Microspores. 



The Sexual Generation [Oophore] which is developed from th^ spore always 

 preserves, in Vascular Cryptogams, the form of a thallus ; it never attains, as in the 

 more highly developed Mosses, to a differentiation into stem and leaf, but remains 

 small and dehcate, and closes its life with the commencement of the development of 

 the second generation. It appears, therefore, externally as a mere precursor of further 

 development, as a transitional structure between the germinating spore and the 

 variously differentiated second generation. Hence the name Prothallm?n has been 

 given to this first or sexual generation of Vascular Cryptogams. 



In the Ferns and Equisetaceae the prothallium resembles the thallus of the 

 lowest Hepaticae. The prothallia sometimes continue to grow for a consider- 

 able time ; they contain a large amount of chlorophyll, and form numerous root- 

 hairs. After they have thus attained sufficient vigour by independent nourishment, 

 they produce archegonia and antheridia, usually in considerable numbers. A 

 tendency to become dioecious is sometimes manifested in these prothallia, although 

 they proceed from similar spores; both kinds of sexual organs being, however, 



c c 



