AS SEEN IN .SEED-PLANTS 97 
show that sterilisation of aporemenious cells is not. uncommon in the anthers 
of Seed-Plants. 
Evidence of sterilisation is also found in the ovules of Seed-Plants. 
Among the Gymnosperms, the Gnetaceae show an archesporium consisting 
of a group of hypodermal cells: in Gxetum Gnemon, which is the best 
known example, these give rise to a considerable mass of sporogenous cells, 
but only one embryo-sac is finally matured. Inthe. Cycads the case is 
similar, inasmuch as there is a considerable tract of sporogenous tissue, 
though only one embryo-sac matures. In the Coniferae also there is 
frequently a multicellular archesporium, and several embryo-sac-mother-cells 
have been seen to enlarge in Zaxus and Seguoza, but in most of them 
only a single one. Among Angiosperms a condition very similar to that 
1a pe 
Cares 
DE 
PS 
Fic. 54. . 
h 
A, longitudinal section of one loculus of a young stamen of £; idium 
showing differentiation of the potential archesporium into fertile cells (2) and sterile 
cells (s). , similar section of stamen of Clarkia elegans, more advanced, showing a 
sterile septum dividing the contents of the single loculus into distinct sporogenous groups 
(sf). ¢=tapetum. X 365. 
in Gnetum is seen in Casuarina (Figs. 55, 56): this case is particularly 
interesting, since the potential embryo-sacs are not simply obliterated by 
the growth of the favoured one, but some develop into tracheides with 
thickened walls—a proof that permanent sterile tissue may be formed from 
potentially sporogenous cells. In certain Amentiferae also a_ similar 
formation of tracheides has been seen. “A multicellular archesporium is 
‘common, besides, in other Archichlamydeous Dicotyledons, eg. in the 
‘Ranunculaceaé and Rosaceae, and some others (Fig. 57); but it is 
apparently less common in the more advanced Dicotyledons and in the 
‘Monocotyledons. The examples thus quoted suffice to show that sterilisation 
of potentially sporogenous cells is frequent both in the microsporangia 
‘and in the megasporangia of the Seed-Plants. 
And thus it is seen that evidence of sterilisation is widespread: it is 
found in all the main groups of the characteristic Flora of the land, both 
G 
