STERILISATION 



of demonstration in a fossil. The general conclusion may be drawn from 



such cases as those cited, that sterilisation has played a considerable part 



in the sporangia of Pteridophytes. 



In Seed-Plants also there is frequent evidence of sterilisation of cells 



of a potential archesporium, both in megasporangia and in microsporangia. 



In the latter, examples have been seen in which a considerable proportion 



of the cells of the sporogenous group are 

 obliterated in much the same way as in Psilotum, 

 But in the anthers of not a few Angiosperms 

 partial or complete septa of sterile tissue may 

 be formed in plants whose near allies have 

 their pollen-sacs non-septate. Thus, in the 

 Onagraceae the stamens of most of the genera 

 are of the ordinary quadrilocular type; but in 

 the genera Circaea, Gaura, Clarkia^ and 

 Eucharidium the four loculi are each divided 

 transversely by one or more sterile septa : these 

 septa may consist of only a single layer of cells 

 having the character of tapetum, or of two 

 layers, or even of four or more, of which the 

 middle layers then resemble the tissue of the 

 connective. An examination of early states of 

 development of these anthers shows that the 

 septa result from sterilisation of part of the 

 sporogenous tissue, for in sections it is seen 

 that the sporogenous cells and those which will 

 form the septa originate from a common layer 

 corresponding to the archesporium of normal 

 anthers of the family (Fig. 54). A similar 

 state of things has been described in certain 

 of the Mimoseae (Inga, Calliandra, Acacia, 

 Albizzia], in many of which there are eight 



. ' c , , , c 



pollen-sacs in place of the normal number of 

 four ; while in others (Parkid) the number 

 may be much larger. Here, again, the 

 developmental history shows that sterilised 

 archesporial tissue provides the septa which 

 divide the four original pollen-sacs into eight 



or more loculi. With these may also be compared the cases of V^sc^^m 

 and Loranthus. Developmental study of the anther of Rhizophora 

 has given the same result : in its massive anther the small pollen- 

 sacs are very numerous, distributed over a large surface : Warming has 

 concluded that the anther became multilocular by the arrest of the 

 further development of certain parts of the pollen-forming tissue (see 

 Fig. 72, p. 142). Such examples, which by no means exhaust the list, 



FIG. 53. 



Part of a section of a megaspor- 

 angium of Isoetes. The cell marked 

 (m) is the only fertile spore-mother- 

 cell, the rest are undergoing vegeta- 

 tive divisions, including the cell (a) 

 as shown by other sections of the 

 series. Thus sterilisation affects the 

 large majority of the cells of the 

 sporogenous group. X245- (After 

 Wilson Smith.) 



