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 
Fic. 43. its sporogenous group. When this. 
Botrychium daucifolium, Wall. Sporangium in vertical 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 archesporium (Fig. 44). The same 
section, with the sporogenous tissue shaded. 200. 
terms are also applied in the case of the 
Bryophyta, the chief difference being that in 
them the sporogenous tissu 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 Psi/otum (Fig. 45). It will be 
well to consider carefully how such a case as 
this is to be regarded from an_ evolutionary 
point of view, for it will be seen later that the 
same reasoning as is used in the present case 
Fic. 44. 
Botrychium daucifolium, Wall. 
Early stages of development of spor- 
angia, showing by comparison that the- 
sporogenous group originates from a 
single ‘‘archesporial” cell. xX 200. 
is applicable to a great number of others also. Adequate investigation 
shows that in the sporangium of Psz/otum all the cells of the sporogenous. 
