372 PART III. THE CLASSIFICATION OF PLANTS. 



GROUP III. 

 PTEBIDOPHYTA (Vascular Cryptogams). 



The distinguishing characteristics of the plants forming this 

 group are the following : The life-history presents a well-marked 

 alternation of generations, as in the Bryophyta ; but here it is the 

 sporophyte which is the more conspicuous form, constituting " the 

 plant." The sporophyte becomes quite distinct from the gameto- 

 phyte at an early period : it is differentiated (with but few 

 exceptions) into root, stem, and leaf ; and in all cases it contains 

 well-developed vascular tissue. The gametophyte, generally 

 termed the prothallium, is a relatively small thalloid body, usually 

 short-lived, containing no trace of vascular tissue. 



The group includes the three classes, Filicina3, Equisetinae, 

 Lycopodinse. 



The SPOROPHYTE is developed from the oospore : its embryogeny 

 is direct and holoblastic. The oospore undergoes division, in all 

 cases, into an epibasal and a hypobasal half, by a basal wall which 

 is either more or less nearly parallel to the long axis of the 

 archegonium (Leptosporangiate Filicinae) or more or less nearly 

 transverse to it : the epibasal half usually faces the neck of the 

 archegonium, but in the Lycopodinae the hypobasal half occupies 

 this position. In the Filicinae and Equisetinae, the formation of 

 the basal wall is followed by the formation of another wall at 

 right angles to it (quadrant-wall) so that the embryo now consists 

 of four cells which are quadrants of a sphere, and this is followed 

 by the formation of a third wall (oclant-ivall}, at right angles to 

 both the preceding, so that the embryo now consists of eight 

 cells which, are octants of a sphere. In the Lycopodince the 

 segmentation leading to the formation of quadrants and octants is 

 confined to the epibasal half, the hypobasal half remaining un- 

 divided or undergoing a few irregular divisions. From the 

 epibasal half, the primary stem and one or two primary leaves 

 (cotyledons) are developed in all cases. The hypobasal half 

 gives rise, in the Filicinae and Equisetinae, to the primary root 

 and to the foot, with but few exceptions (e.g. Salvinia in which 

 there is no primary root) : in the Lycopodinae the hypobasal half 

 gives rise to a suspensor, an organ homologous with the foot of the 

 Bryophyta. 



The foot (as also the suspensor : see p. 14) is an embryonic 



