94 



STERILISATION 



of the arrested cells is sometimes simply nutritive, sometimes they form 

 permanent tissue-masses. In Lycopodium and Phylloglossum, and in the 

 ( homosporous Ferns, after the sporogenous tissue is first defined, all its 

 cells normally undergo the tetrad-division, and develop spores : occasional 

 cells may become disorganised without full development, though, as a 

 rule, all the potentialities are realised. But among the Marattiaceae, 

 where, as a rule, all the cells of the sporogenous groups are fertile, a number 

 of exceptional cases have been noted : the most remarkable is that of 

 Danaea, in which it has been shown how certain cells of the larger sporo- 

 genous groups remain sterile, and may be develope^d as tapetum, or even 

 as component cells of a partial septum (Fig. 49). Somewhat similar 

 conditions have been seen in Kaulfussia, Marattia, and Angiopteris* 



FIG. 50. 



A, apex of sporangium of Equisetum litnosum, L., showing the sporogenous cells, 

 surrounded by the tapetum (shaded), and sporangial wall. F>, shows part of an older 

 sporangium with its tapetum (t) still clearly defined, though the individuality of the cells 

 is lost : within this the sporogenous tissue, of which certain cells (a) are abortive. 



X200. 



In other cases it has been found that only a portion of the cells of the 

 sporogenous group are fertile, as already described for Psilofum (Fig. 45) : 

 this has been shown to be the case also in Tmesipteris, and in Equisetum 

 (Fig. 50), and it has been described also as an occasional feature in the 

 Ophioglossaceae. In all of these a varying proportion of the sporogenous 

 cells are sterile, and become disorganised without forming tetrads. As 

 the proportion of the sterile to the fertile cells is not fixed, an elastic 

 arrangement exists which leads to the largest number of spores being 

 brought to maturity that the plant at the time can support. The limits 

 of the sporogenous tissue in early stages are difficult to define in these 

 large sporangia, and they show considerable irregularities : this is especially 

 so in Psilotum, Tmesipteris, and Ophioglossiim, and it appears to be partly 

 due to the ill-defined and broad tapetum which is formed peripherally,. 



