232 MANUAL OF BOTANY 



as paraphyses (fig. 501). In the latter the sexual organs are 

 generally confined to a thickened portion of the prothallium, 

 extending centrally along the under side and known as the 

 cushion. 



In such of the Algse as show a differentiation of their body 

 into stem and leaf, the gametangia are foliar in their origin, and 

 are borne upon special leaves. In Fucus and its allies, in which 

 the plant body is a thaUoid shoot, there are special terminal 

 collections of /them, each being formed of a number of almost 

 closed depressions or pits, known as conceptacles [figs. 5 and 

 493). In some species each oonceptacle contains both anthe- 

 ridia and oogonia, in others only one or the other kind. 



In consequence of the generally slight morphological differ- 

 entiation of the gametophyte as compared with the sporophyte, 

 we find a somewhat different distribution of the reproductive 

 organs in the two cases. As we have seen, the sporangia are 

 usually borne upon leaves which may be highly specialised or 

 may be almost indistinguishable from the foliage leaves. The 

 occurrence of axial sporangia is much less common, though, as 

 we have seen, it is met with in many very diverse groups. On 

 the other hand, the gametangia are much more generally axial 

 in origin, rarely being found on leaves, and then on those which 

 are not highly specialised. 



In the Mosses and Ferns and their allies the sporophyte is 

 at its origination always attached to the gametophyte, in con- 

 sequence of the zygote germinating without being set free from 

 the archegonium. The spore is, however, always detached from 

 the sporophyte before germination. This is not the case in the 

 Phanerogams, where from the mode of its development the 

 megaspore always remains in the sporangium. The microspore 

 in these plants is, on the other hand, always set free. 



As we have thus cases in which the gametophyte, which 

 bears the female cell, is developed in the spore while the latter 

 remains in the sporangium, and others in which the spore escapes 

 before the gametophyte arises from it, we find two methods of 

 fertilisation which are characteristic of those respective groups. 

 Where the spore produces the gametophyte after liberation 

 from the sporangium, fertilisation takes place by a free-swimming 

 antherozoid making its way to an archegonium, down the neck 

 of which it passes to the oosphere. Where the megaspore 

 remains attached to the plant its prothallium or gametophyte 

 is inaccessible to antherozoids, and these are therefore not 

 developed. The microspore is carried by various agencies to the 



