9 o 



STERILISATION 



spore-production before the tetrad-division. It is a secondary matter 

 morphologically that in this case their existence is brief; but physiologically 

 it is important, for they are sacrificed to furnish better nutrition to the 

 others which remain fertile, and produce spores. The structure seen in 

 Fig. 45 is thus to be interpreted as indicating the sterilisation of certain of 

 the potentially fertile cells in the sporogenous group of Psilotum. 



A second example illustrating this partial sterilisation of a sporogenous 

 group may be quoted : it is selected from among the Bryophytes ; but 

 the same arguments as in the previous case will equally apply here. 

 Fig. 46 illustrates two stages in the development of the sporogonium of 

 Aneura : the younger shows the clearly defined, hemispherical internal group 



FIG. 46. 



A, median section of young sporogonium of Aneura ambrosioides. The internal mass 

 of cells of the sporogonial head ("archesporium ") is already differentiated so as to 

 indicate the sterile elaterophore, and the outer fertile region. B, the same, older : the 

 indications of sterilisation have extended outwards, and it is only the peripheral fringe of 

 cells (shaded) which will be sporogenous. C, transverse section of the'same. X 150. 



of cells of the sporogonial head, which are equivalent inter se, inasmuch 

 as they have a common origin ; but they are already differentiated into two 

 distinct regions, a peripheral fertile region, of which the cells are shaded, 

 and a central sterile region. The former differentiates at a later stage into 

 spore-mother-cells and elaters : the latter forms the sterile elaterophore. 

 The whole hemispherical group corresponds in position to the body 

 similarly placed in other Hepaticae, which have no elaterophore, and 

 in which the whole region develops into spores and elaters. This case 

 therefore illustrates an advanced stage of sterilisation of tissue which may 

 be held to be ancestrally sporogenous throughout. ' But the final fate of 

 the sterile cells here is not merely to serve as evanescent nutritive cells; 

 for the elaterophore and elaters are a permanent tissue and permanent 

 cells, which remain till the ripeness of the spores, and are functional in 

 their dispersal. 



