106 Oliver . — On Physostoma elegans , Williamson , 
in Physostoma the individual bundles begin to diverge much lower down. 
It is difficult to resist interpreting the former as the derived, the latter as 
the relatively primitive condition. 
2. The Nucelhis. 
The investigation of Physostoma has brought to light a number of 
features connected with the structure of the nucellus which cannot be dis- 
missed without consideration. 
These include (a) the zone of secretory sacs; (b) the tapetum; (c) the 
form of the megaspore cavity, and the pollen-chamber. 
(a) The Zone of Secretory Sacs. That the secretory zone should be 
referred, morphologically, to the province of the nucellar wall rather than 
to that of the integument, seems to follow clearly from the non-continuation 
of the secretory zone into the free integumental arms. At the place of 
separation, the bundles and ground-parenchyma pass out into the arms, 
whilst the secretory zone and tapetum pursue their course into the free 
nucellar apex. 
In view of the general similarity in organization that obtains between 
Physostoma and the other members of the Lageuostcma-gmup of seeds, 
it is of some interest to note that in none of the latter have secretory sacs 
been observed in the nucellus. For if it be true, as we conjecture, that the 
whole of this group of seeds belonged to separate but as yet undiscriminated 
members of the Lyginodendreae, we have in Physostoma the sole representa- 
tive of the group of seeds which retains in a noticeable way a character that 
is invariably present in the vegetative organs of the hypothecated seed- 
bearers. As an isolated feature, not much significance would attach to 
the point ; but it assumes a different complexion when we find it asso- 
ciated with an ensemble of peculiar and primitive characters, as we do in 
Physostoma. 
(b) The Tapetum. As regards the tapetum, so striking a feature in 
Physostoma , little need be said. Layers of this kind, occupying a position 
between the sporangial wall and the sporogenous complex, are generally 
present in the dehiscent sporangia of the archegoniate or spermophytic 
series, whilst they have been frequently met with in the indehiscent sporangia 
(ovules) of both Gymnosperms and Angiosperms. Notwithstanding the 
doubt that has been expressed as to whether these structures can be regarded 
as really homologous, in view of the diversity of their development 1 in 
different cases, the presence of an extensive tapetum in an early seed like 
Physostoma raises the question whether it may not be a structure inherited 
from some non-spermophytic ancestor. Data bearing on this point in the 
oldest known Pteridophytes— the Botryopterideae — are meagre in the ex- 
treme, but so far as they go they point to the presence of a considerable 
1 Cf. Goebel in Organography of Plants, pt. 2, p. 59 6. 
