104 



THE SPORANGIUM DEFINED 



of the spores. The extent of the wall as such is closely related to the 

 position of the sporogenous cells : where these are deeply sunk, the wall 

 is a mere roof over them : where they are carried outwards by the growth 

 of the tissues surrounding them, and a projecting sporangium is formed, there 

 the wall envelopes them as a tissue of greater extent. But in either case 

 it is continuous with the tissue of the sporangium-bearing part, of which it 

 is to be regarded as a specially developed region. This view of it accords 

 well with the structure of sunken sporangia, as in Ophioglossum (Fig. 59), 

 in which the tissues of the wall are continuous with and little differentiated 

 from the cognate tissues of the spike. The same is the case with other 

 Ophioglossaceae, even in those where the sporangia project ; but in these, 

 as also in the Lycopods and Equiseta, there is some further specialisation 



of the wall for dehiscence than 

 in the deeper-seated forms. In the 

 Filicineae still more exact specialisa- 

 tion is the rule, and the mechanical 

 nninihts becomes a marked feature 

 in the stalked sporangium. But a 

 comparison of the Marattiaceae and 

 other Ferns leads to the conclusion 

 that the presence of an annulus, 

 and its elaborateness, are to be 

 correlated with the freedom of the 

 sporangium from mutual relations 

 with other bodies. The annulus, 

 in fact, is still only a specialised 

 region of the sporangial wall. This 

 conclusion accords with the facts 

 F|<; - 59- of its inconstancy, for sporangia 



reticuiatnm, L. Vertical section of which open under water may have 



the deeply sunk sporangium, with the sporogenous tissue . . . 



shaded, x 100. no such mechanism. I hat is the 



case in Isoetes, while the sporangia 



of the Hydropterideae are also entirely without an annulus ; nor is there 

 any stomium in them, which would localise dehiscence. Loxsoma is a 

 specially interesting case, for there the annulus, though indicated by the 

 cell-divisions as complete, is only partially indurated : as a matter of fact, 

 the part of it which is not indurated could not possibly be mechanically 

 effective, owing to the mode of packing of the sporangia in the sorus 

 (Fig. 60). 



The general conclusion to be drawn is, that while the protective wall 

 itself is always present, those mechanical arrangements collectively desig- 

 nated by the term "annulus" are not essential or constant parts of the 

 sporangium, however constant they may seem to be in certain groups: 

 where they cannot be mechanically effective they may be entirely omitted. 



Nor is the tapelum to be looked upon generally as a morphological 



