CHAPTER XIII 



THE SPORE-PRODUCING ORGANS {contiimed) 



(B) THE SPORANGIUM 



The sporangium of Dryopteris has already been described as a fair example 

 of a type common in Leptosporangiate Ferns. Its parts when mature are 

 the stalk of attachment, the distal capsule with thin lateral areas of wall, 

 and the indurated annnlus, and stomium. Within are the spores, dry and 

 dusty when mature, but surrounded during development by the nutritive 

 tapettim. Each whole sporangium of Dryopteris springs from a single 

 superficial cell, which after segmentation according to a regular plan gives 

 rise to the several parts. At the centre of the young sporangium a single 

 cell may be recognised at an early stage as the archesporiiim, from which, by 

 further divisions, the tapetiiin and, finally, the spore-mother-cells are derived. 

 The latter are twelve in number and, undergoing the tetrad-division, they 

 form the definite number q{ forty -eight spores (Chapter l). 



The characters thus seen constantly in the sporangia of this Fern are 

 nevertheless very variable within the Class of the Filicales. The sporangium 

 may originate not from one cell but from a group of cells (Eusporangiate 

 Ferns). The stalk may be absent, and the sporangium may then be deeply 

 immersed in the tissue of the sporophyll {Ophioglosstim). The sporangial 

 wall may consist not of a single layer of cells, as it is in Dryopteris, but 

 of many layers, as in BotrycJiiuvi or Angiopteris. The opening mechanism 

 may appear much more massive and complicated than in Dryopteris : in 

 some cases there may appear to be no opening mechanism at all (Hydro- 

 pterideae). The tapetum may not be represented by any definite band of 

 cells {Ophioglossum). Finally, the spores may be of a number largely exceed- 

 ing, but sometimes less than the number forty-eight seen in Dryopteris. 

 Frequently the individuality of the sporangia is not maintained : for, as 

 will be shown, fission may result in an increase of their number {Danaea), 

 while in other cases many of them may be coherent so as to form a massive 

 synangium {Marattia, Ophioglossum). It may well be expected that a body 

 of the essential importance of the sporangium, but showing such a high 

 degree of variation, should provide characters of the greatest value for com- 

 parison and for the phyletic treatment of the Class. And the fact that their 

 features are, relatively constant for any given species, genus, or even larger 

 group, makes them all the more valuable for this end. 



