46 Oliver and Salisbury . — On the Structure and 
face of the seed. The other extreme is afforded by Conostoma, where the 
minute lagenostome lay at the foot of a long and specialized micropyle 
which traversed a canopy in which the unit parts, though more highly 
modified than in Lagenostoma , were still recognizable. 
A peculiar organ, the plinth, claims special attention in Conostoma , not 
merely from its dimensions, but also on account of the part which it played 
in the reception of pollen. Though this zone or region is represented in all 
three types, it is only in Conostoma , and the probably related Gnetopsis , 
that it attained to any special significance. To what extent the nucellus of 
existing Gymnosperms — especially Cycads — may have undergone analogous 
elaboration cannot be stated with any confidence owing to the defective 
state of our knowledge of the developmental history of the ovules. 
The ribbing and angling of these seeds also raises matters of interest 
dealt with in the body of the paper (p. 41). In these ribs there appear to 
be presented traces of what may be regarded, in the light of Physostoma, as 
the original segments or lobes of the ancestral integument. As these show 
considerable variety, even in allied seeds, in the relative prominence and in 
the presence or absence of accompanying vascular strands, it is evident that 
no great reliance can be placed on these characters for diagnostic purposes 
— especially where the larger groups are concerned. Incidentally, it may 
also be remarked that incipient stages in the passage from radial to bilateral 
symmetry appear to be illustrated by both Conostoma oblongU 7 n and Gne- 
topsis elliptica. This shows, if further proof be needed, that the old pro- 
visional distinction of palaeozoic seeds into radiospermic and platyspermic 
types had little or no significance as a guide to affinity. 
Though allusion has been made to the modification and elaboration in 
different directions which the seed underwent, it would be premature hastily 
to suppose that our different types had necessarily diverged from a common 
seed-possessing ancestor. In these days when the doctrine of polyphylesis is 
steadily gaining ground, the alternative view that (to take a concrete case) 
Physostoma , Lagenostoma , and Conostoma had been separately derived from 
as many related but distinct cryptogamic types will certainly have to be 
considered. On that view, then, the differences between our seeds would 
depend not only on such divergences as arose after the establishment of the 
seed habit, but they would be, in part at least, determined by inherited 
differences already present (or latent) in the several ancestors. 
Moreover, the coming of the seed habit must, from the evolutionary point 
of view, have marked a relatively active period ; for, even if we suppose the 
qualifications for seed-bearing to have been acquired in cryptogamic days, 
there must have been a transitional period during which the less immediately 
serviceable portion of the cryptogamic inheritance was either eliminated or 
underwent functional change. 
These considerations may perhaps serve to indicate some of the diffi- 
