47 8 BOTANY OF THE LIVING PLANT 



Hon as the sporophyte of Vascular Plants. The end is the same 

 for both, viz. to develop on land as large a vegetable system as 

 possible, 'so as to provide material for the largest possible number 

 of germs. The one phylum has solved it by enlargement of the 

 sporophyte, which thus becomes the substantive " Plant " of 

 vascular types. The other has solved it by elaboration of the 

 gametophyte, which has similarly become the substantive " Plant " 



of the Bryophytes. 



For a less condensed treatment of the Bryophyta see Primitive 



Land Plants, Chapters I. to VI. 



The Psilophytales. 



Till recently it had been thought that among Plants of the Land the widest 

 gap of organisation lay between the Bryophyta and the Pteridophyta, not- 

 withstanding that the life-cycle of both is essentially the same. Even now 

 the distinction seems wide when only living examples of them are compared. 

 But in late years there have been discovered from rocks of Devonian Age, and 

 widely spread geographically, fossils grouped as a new Class of the Psilo- 

 phytales. Their sporophytes only are known, but these are clearly vascular 

 land-plants of rudimentary organisation. They are rootless, and the simplest 

 of them leafless. The general character of two of the best known types is 

 shown in Fig. 372 a, which represents reconstructions of them after Kidston and 

 Lang. Hornea, the smaller, has a tuberous base attached within the soil by 

 rhizoids. From this a bifurcating cylindrical shaft rises erect and leafless. 

 Some of its finer branches bear terminal sporangia. The larger plant, Asteroxy- 

 lon, has dichotomously branched leafless rhizomes without absorbent hairs : 

 their finer branchlets ramify in the peat in which the plants grew. From these 

 sprang large branching and erect trunks. They had localised growing points, 

 and bore simple microphyllous leaves ; but these were absent from the smaller 

 distal branchlets which dichotomised freely, and bore terminal sporangia. 

 The structure of these plants is very completely known. They are quoted 

 here as examples of early vegetation of the land, which offer illuminating 

 comparison on the one hand with the sporogonia of the Bryophytes, and on 

 the other with the sporophytes of the simpler types of Pteridophytes previously 

 known. 



There are obvious restrictive defects in the organisation of the Mosses and 

 Liverworts, which account for their dwarfed habit. In the sporophyte we note 

 its physiological dependence on the gametophyte, and the absence of continued 

 apical growth and branching. ' But the diploid phase of vascular plants is 

 habitually fixed in the soil and diffuse in form : these features appear in rudi- 

 mentary outline in the very ancient Psilophytales. Hence we may rightly see 

 in them types which suggest that varied advance which characterises the 

 Pteridophytes, and vascular plants generally. Without suggesting close 

 affinities by descent with any of these, the early existence of the Psilophytales 



