208 



THE SPORE-PRODUCING ORGANS 



[CH. 



condition. It thus appears that the sorus cannot be held as a definite 

 morphological unit for all Ferns. It is only a congeries of spore-producing 

 units. The essential spore-producing organ is the Sporanginni itself. 



Constitution of the simple Sorus 

 The simplest way in which the sporangia are borne is singly, and their 

 position is then either terminal or marginal. If the term sorus be here 

 applied at all, each sporangium would be a "monangial sorus." This is seen in 

 primitive living types, and in certain fossils. The solitary distal sporangium of 

 Staiiropteris is an example (Fig. 197). Among living Ferns Botrychiiim, one 

 of the Ophioglossaceae, and Mohria, among the Schizaeaceae, show isolated 

 sporangia, and it is notable that in these families their position is marginal, 

 or we might describe them as distal on the tips of the more or less webbed 

 branchings. In such cases the sporangia themselves are relatively large 

 (Fig. 198), while their position on a vein-ending 

 secures their effective nutrition. But in many 

 early fossils, and in most primitive living Ferns, 

 many sporangia are associated together forming 

 groups, each seated in the same way on a vein- 

 ending. Often they are arranged rosette fashion, 

 round a central receptacle. This constitutes the 

 radiate-uniseriate type of sorus. It is seen in a 

 rudimentary state in Zygopteris, where the spo- 

 rangia form distal tassels (Fig. 199). It is charac- 

 teristic of the Gleicheniaceae (Fig. 200), Matonia, 

 and the Marattiaceae ; also of many early fossils, 

 such as Corynepteris, Scolecopteris, AsterotJieca, etc. 

 Sometimes the sporangia are united laterally 

 so as to form a coherent synangium. This is 

 illustrated in the fossil PtycJwcarpus nnitus (Fig. 201), and in Marattia and 

 CJiristensenia among living Ferns. The confluent sporangia of Ophioglossuni 

 may also be quoted as an example of essentially the same phenomenon, 

 which may be held as derivative. It probably owed its origin, and finds 

 its biological justification, in the mutual protection which the sporangia 

 thus acquire. 



Occasionally a reduction in number of the sporangia in the radiate- 

 uniseriate sorus may be traced by comparison of related species. For instance, 

 in Gleichenia flabellata, the basal sori of the pinnule may have five or six 

 sporangia ; but the distal sori only three (Fig. 200). In § En-gleichenia the 

 number is usually two in each sorus, sometimes only one. It seems probable 

 that this low number is the result of reduction in these attenuated xerophytic 

 Ferns. On the other hand, by an elongation of the sorus along the course 



Fig. 198. Botrychium Lunaria. 

 Part of the spike, with open 

 sporangia. (After Luerssen. ) 

 Enlarged. 



