380 PART IV. CLASSIFICATION. 



the first anteriorly in the middle line, the second and third one on 

 each side of the first. If none of these become fertilised, the pro- 

 thallium resumes its growth both in breadth and length, and a 

 second row of archegonia is formed in front of the first. Similarly 

 a third and a fourth row, with an increased number of archegonia 

 in each (seven or more), may be successively produced. In Pilu- 

 laria also the prothallium grows to a considerable size if the arche- 

 gonium is not fertilised, though no more archegonia are formed. 



The female cell, or oosphere, developed in each archegonium, does 

 not require any special description. 



Order 7. Salviniaceae : microsporangia and macrosporaiigia in distinct 

 sori, each sorus being covered by an indusium ; the spores are surrounded 

 by spongy mucilage, forming an episporium or perinium round the in- 

 dividual macrospores, and holding all the microspores together within the 

 sporangium (Salvinia) or in several groups or massulse (Azolla) ; the female 

 prothallium is relatively large and bears several archegonia. 



No species of Azolla is European: Salvinia natans occurs in Southern 

 Europe. 



OrderS. Marsileaceae : microsporangia and macrosporangia in the same 

 sorus, a number of sori being enclosed in the specially modified sporo- 

 phyll-segment, the whole forming a sporocarp ; each spore is invested by 

 a mucilaginous prismatic perinium : the female prothallium is relatively 

 small, and bears but a single archegonium. 



Pilularia globulifera is the one British species belonging to this order. 

 The European species of Marsilea are M. pubescens, which occurs in the 

 Mediterranean region, and M. quadrifoliata, in Central Europe. 



CLASS VI. EQUISETIN^. 



This class includes, among existing plants, only the homosporous 

 order, Equisetacese ; but there are many extinct fossil forms, some 

 of which are undoubtedly heterosporous. 



Order 1. Equisetaceae. This order includes the single genus Equisetum 

 (Horsetail). Of the twenty species of this genus, about half are British (E. 

 arvense, maximum, silvaticum, paluntre, limosum, hyemale,trackyodon,varieya- 

 tum, pratense, iitorale). 



THE SPOUOPHYTE. The utern consists of a horizontal, subterranean, much- 

 branched rhizome ; some of the branches come to the surface, and are the 

 sub-aerial shoots. The rhizome and its branches are very distinctly seg- 



