AS SEEN IN SEED-PLANTS 97 



X 



show that sterilisation of sporogenous 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 Gnetum 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. In the 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 Taxus and Sequoia^ but in most of them 

 only a single one. Among Angiosperms a condition very similar to that 





FIG. 54. 



A, longitudinal section of one loculus of a young stamen of Rucharidium concinuum, 

 showing differentiation of the potential archesporium into fertile cells (a) and sterile 

 cells (j). , similar section of stamen of Clarkia elegans, more advanced, showing a 

 sterile septum dividing the contents of the single loculus into distinct sporogenous groups 



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, e.g. in the 

 Ranunculaceae and Rosaceae, and some others (Fig. 57); but it is 

 apparently less common in the more advanced Dicotyledons and in the 

 Monocotyledons. The examples tfius 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 



