96 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 Pstlotum. 
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 Gaura, Clarkia, and 
LEucharidium 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 (luga, Calliandra, Acacia, 
Alvizzia), in many of which there are eight 
Circaea, 
Fic. 53. 
Part ofa section of a megaspor- 
angium of /soetes. 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. 245. (After 
Wilson Smith.) 
pollen-sacs in place of the normal number of 
four; while in others (Parka) 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 Viscum 
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, 
