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 stalk, which supports the sporangial head; the latter consists 

 of the wall, with an opening-mechanism, the annulus ; within the wall at 

 an early stage is the archesporium ; later, the tapetum is differentiated, which 

 surrounds the sporogenous 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 wall: 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 



