350 Bower . — A Theory of the 
are found in the large sporangia of Lepidostr obits Brownii , 
while in other types of Lepidostr obus^ as already shown by 
Professor Williamson, the sterile masses take the form of 
longitudinal plates, which project far up into the cavity of the 
sporangium in the mature state. Unfortunately it is impos- 
sible to follow the development in these cases, but I can assert 
that in point of position and structure, these processes, when 
mature, are strikingly similar to the trabeculae of Isoetes. 
Turning for a moment to the megasporangia, the arrest of 
potential sporogenous cells is there a prominent and well- 
known fact. For instance, in Setaginella a single tetrad of 
megaspores is developed, and all the other sporogenous cells 
are arrested. A similar condition is found in other hetero- 
sporous Pteridophyta. Again, in Phanerogamic plants there 
is frequent evidence of sterilization of cells of a potential 
archesporium : among the Gymnosperms Gnettim is a well- 
known example, demonstrated first by Professor Strasburger. 
Again, among the Angiosperms a similar condition is found 
in the ovule of Castiarina : 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 condition has recently been 
demonstrated. Lastly, I would recall the case of Rosa livida , 
also described by Professor Strasburger. 
It is thus seen that in megasporangia and ovules, steriliza- 
tion of potential sporogenous cells is of frequent occurrence, 
the cells arrested being for the most part obliterated by the 
developing megaspores. In the pollen-sacs of Angiosperms 
a similar sterilization of single cells has been noted, and is, 
I doubt not, common enough. But in the anthers of certain 
Angiosperms there may also be seen a formation of complete 
septa of permanent sterile tissue, dividing the pollen-sacs 
transversely, in plants whose near allies have their pollen-sacs 
not septate. Such examples appear to me to show in the 
most conclusive way that septa may be formed by a partial 
