234 



THE SPORE-PRODUCING ORGANS 



[cii. 



Fig. 233. Part of a transverse section of a fertile leaf of Leptochilus 

 tricuspis, showing the diplodesmic structure, and branched hairs associ- 

 ated with the sporangia. ( x 16.) 



In 



Protection of the Sorus 

 all Ferns protection of the spore-producing organs is in the first 



instance afforded by the juxtaposition of parts in the young state, and this 

 is specially effective as a consequence of the 

 circinate vernation, by which the distal end 

 of each leaf is enfolded within the older parts. 

 The lower parts of the leaf are more exposed, 

 and this may explain the frequent absence of 

 sporangia at the base. Osmunda regalis is an 

 example of this, and it is a common feature 

 in Nephrodiuin. In addition to such general 

 protection, special developments of the most 

 various kind appear in relation to the sorus. 

 When these take definite form as protective 

 organs they pass under the general name of 

 Indii-sium. It will be shown, however, that 

 such organs originate in various positions, and 

 spring from divers sources, so that the term 

 indusium cannot connote any definite morpho- 

 logical entity, but is used only in a general 

 descriptive sense. 



As a rule the Simplices, which form their sporangia simultaneously, initi- 

 ate them early. This fact and their usually massive character make indusial 

 protection less necessary here than it is in the Gradatae and Mixtae, in which 

 more delicate sporangia are produced in a succession spread over a long 

 period. Sometimes there is mutual protection by fusion of the sporangia 

 to form massive synangia, as in Alarattia and OpJiioglossuvi. These features 

 help to explain the usual absence of special protection in the Simplices. A 

 few hairs may be found round the base of the sorus in Angiopteris and 

 Marattia ; in Danaea elliptica the sori are partially sunk into the tissue of 



Fig. 234. Various forms of the sori in 

 Polybotrya. Diagrammatic: after 

 Frau Schumann. 



