2l6 



THE SPORE-PRODUCING ORGANS 



[CH. 



ages may be side by side (Fig. 209). The change to the mixed state thus 

 indicated in Dipteris has been perpetuated in a number of Ferns derivative 

 from the Dipteroideae {Studies, v), such 

 as Leptochiliis tricuspis, Neocheiropteris, and 

 probably Phlebodimn, and others. There is 

 no reason to think that the gradate condi- 

 tion ever intervened in such cases. 



The " mixed " sorus has usually a flat 

 receptacle, and where closely packed the 

 heads of the mature sporangia project be- 

 yond the rest. Even when crowded and 

 laterally compressed their distal end is free 

 (Fig. 208, C). Dehiscence takes place out- 

 wards by means of the vertical annulus. There 

 is abundant evidence that in the course of 

 Descent the oblique annulus has swung 

 towards the vertical plane in the sporangia 

 of the Mixtae, which facilitates the shedding 

 of the spores in their crowded sori. 



There are thus three main types of constitution of the sorus, and the 

 Ferns which show them may be ranked as Siniplices, Gradatae, and Mixtae. 

 Such distinctions cannot, however, be held as affording in themselves any 

 true phyletic grouping, notwithstanding that the Gradate and Mixed sori 

 have undoubtedly been derived from those recognised as Simple. The types 

 of sorus should be held as states or conditions which may severally have 

 been achieved in accordance with biological advantage along a number of 

 distinct evolutionary lines. The phyletic grouping will have to be more 

 broadly based than on this single feature. Nevertheless the facts relating to 

 the construction of the sorus are of the utmost value for phyletic argument. 

 It may be held that the simple sorus is primitive, the solitary sporangium, 

 or monangial sorus, being the most primitive of all. The gradate type is 

 derivative from the simple, and therefore more recent. The mixed sorus is 

 the most recent, and may be traced either directly from the simple sorus, or 

 indirectly from it, through the gradate type. 



Fig. 209. Dipteris conjugata ; young sori 

 showing sporangia of different ages in 

 juxtaposition. j?= younger sporangia, 

 / = parapliyses. ( x 300.) .-J = older 

 stage. ( X 100.) (After Miss Armour.) 



The position of the Sorus 



In most Ferns the position of the sorus is upon the abaxial or lower 

 surface of the leaf: each is seated upon a vein, frequently upon a vein-ending. 

 In a very large proportion of them the sori form two more or less distinct 

 rows, one on either side of the midrib of the pinna or pinnule. This is seen 

 in Polypodium vulgare, Dryopteris Filix-mas, etc. In many other Ferns, 

 however, the position may be either actually on the margin of the leaf, or 



