CHAPTER VIII. 
THE SPORANGIUM DEFINED. 
“THERE are two main types of construction of the sporophyte in Archegoniate 
plants which differ in essential features. In the Bryophyte-type it is a 
body dependent on the gametophyte, without appendages of any sort, 
and with the sporogenous tract as one concrete archesporium, while the 
spores are consequently contained in one non-septate sac. In the Pterido- 
phyte-type it is an independent organism, with roots and foliar appendages, 
‘while the archesporia are discrete and usually numerous: accordingly the 
spores are contained in many distinct pockets: these are the sporangia. 
‘There is no definite indication how the polysporangiate state of the 
Pteridophytes came into existence; but with a view to forming an opinion 
it will be necessary to enquire into the characters of the sporangium, 
“and to separate the essential features from the non-essential. 
In any ordinary sporangium of a Fern the following parts are to be 
‘recognised : the s¢a/R, which supports the sporangial /ead; the latter consists 
-of the wa//, with an opening-mechanism, the amnudus; within the wall at 
an early stage is the avchesporium ; later, the ¢apetwm is differentiated, which 
‘surrounds the sforogenous group; ultimately the spores are matured from 
the latter (compare Figs. 4 to 8). It will be necessary to consider which 
-of these parts are constant in sporangia at large, and which are inconstant 
or only occasional parts. It will be possible thus to arrive at some estimate 
as to which of these are essential and which are merely accessory parts 
-of the sporangium. 
Comparison shows that though sporangia are often stalked, still there 
are many others which are quite sessile, and indeed immersed in the tissue 
of the part which bears them (Fig. 59): this indicates clearly that the 
stalk is not an essential part of the sporangium. 
“In all the Archegoniatae the spore-mother-cells are covered externally 
by the sporangial wad/: this is a protective sheet of cells, which may be 
of variable thickness and structure: it serves the several purposes of protec- 
tion, of nutrition, and in many cases of dehiscence, and of mechanical ejection 
