GTIOUP III. PTERIDOPHYTA: FILICINJ! ; HYDROPTERIDE^. 415 



R 



A. 



The female organ, or archegonium, differs in no essential feature 

 of structure or development from that of the homosporous lepto- 

 sporangiate Ferns. In the Marsileaceae, only a single archegonium 

 is developed ; it arises from a cell in the middle of the upper sur- 

 face of the prothallium ; in Azolla, if the archegonium does not 

 become fertilised, a few more arche- 

 gonia may be subsequently developed; 

 in Salvinia, at least three archegonia 

 are always formed, 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 prothallium 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 in 

 creased number of archegonia in each 

 (seven or more), may be successively 

 produced. In Pilularia also the pro- 

 thallium grows to a considerable size 

 if the archegonium is not fertilised, 

 though no more archegonia are 

 formed. 



The female cell, or oosphere, de- 

 veloped in each archegonium, does not require any special de- 

 scription. 



Order 7. Salviniaceae : microsporangia and macrosporangia in distinct 

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

 spongy mucilage, forming an episporium or perinium round the individual 

 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. 



Order 8. Marsileaceae : microsporaugia and macrosporangia in the same 

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

 segment, the whole forming a sporocarp. Each spore is invested by a mucila- 

 ginous 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 Mediter- 

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



Fro. 270. Gametophyte of Sal- 

 vinia (x 60). A Macrosporanginin 

 m with a germinated macrospore sp 

 (dotted outline) ; pt female pro- 

 thallium projecting from the apex 

 of the spore; ar archegonia. B A 

 microspore isolated from a micro- 

 sporangium : sp spore ; pt male 

 prothallium ; a antheridia. 



