88 



STERILISATION 



in the later stages by their large nuclei. In Vascular Plants each sporo- 

 genous group is as a rule derived by division from a single cell, or a row 



or sheet of cells, and in favour- 

 able examples it may present the 

 appearance of a compact mass of 

 tissue, which may readily be re- 

 ferred in origin to a single parent 

 cell (Fig. 43). But the details of 

 those divisions which result in 

 the sporogenous group show great 

 diversity in different plants, while 

 in a not inconsiderable number of 

 cases the limits of the sporogenous 

 group are not found to be strictly 

 coterminous with the tissue-pro- 

 ducts of definite initial cells. It 

 is only by a careful study of the 

 successive stages of development 

 in each individual type of spore- 

 producing organ that it is possible 

 to assign the limits of origin of 

 its sporogenous group. When this 

 has been done, and the genetic 

 story completely made out, the 

 cell or cells which are found to be the ultimate parent-cells of a single 

 sporogenous group are designated its archesporiu m (Fig. 44). The same 

 terms are also applied in the case of the 

 Bryophyta, the chief difference being that in 

 them the sporogenous tissue of each individual 

 forms only one concrete group. 



In not a few cases the whole product of 

 the archesporium becomes converted into spore- 

 tetrads, and ultimately into spores ; but this is 

 not always so. It frequently happens that in 

 the course of development certain cells which 

 spring from the archesporium are diverted to 

 other functions than that of direct spore-pro- 

 duction ; a good instance of this is seen in the 

 sporangium of J'silotnni (Fig. 45). It will be 

 well to consider carefully how such a case as 



Botrycnmm aauci/olium, Wall. 



this is to be regarded from an evolutionary Early stages of development of spor- 

 angia, showing by comparison that th< 



point of view, for it will be seen later that the -| genous group originates from a 



sin^lr " archesporial " cell. X 200. 



same reasoning as is used in the present case 



is applicable to a great number of others also. Adequate investigation 



shows that in the sporangium of Psilotum all the cells of the sporogenous. 



Botrychium daucifolhim, Wall. Sporangium in vertical 

 section, with the sporogenous tissue shaded. X 200. 



FIG. 44. 



