




















24 THE BRITISH FERNS. 
purpose of affording discriminating generic characters in some 
modern systems of classification. 
The nature and position of the receptacle appears to us, indeed, 
to be, theoretically, of considerable importance in a systematic point 
of view, as being the point of contact between the vascular and repro- 
ductive systems of the plants. Practically, it does prove of some 
importance in cases where the fructification is otherwise not easily 
made out. Thus, for example, in the genus Oheilanthes the indu- 
sium is often linear and continuous, quite like that of Pteris, the 
spore-cases by confluence often appearing to be continuous also; 
consequently, unless the actual attachment of the spore-cases is 
taken into account, there is no determinate limit between these 
genera. But in Pteris proper there is a linear continuous receptacle, 
transverse to the general course of the veins, while in Cheilanthes 
the receptacles are punctiform at the tips of the veins. The sori are 
consequently not really, but only apparently linear in the genuine 
species of Chielanthes, and they are thus widely separated from Pteris, 
which otherwise they seem to resemble. This being so, the inter- 
polated genus Pellea is unnecessary. So in Platyloma, the sorus is 
a linear mass of spore-cases, broad indeed, but apparently at the 
margin, and with what appears to be a narrow marginal indusium. 
The species are hence sometimes mistakenly referred to Pteris, and 
they are, indeed, generally classed in the group of Pteridew. But 
instead of having the linear transverse marginal receptacles of Pteris, 
the receptacles of Platyloma are really set in lines the other way, 
that is, extending from the point of each vein more or less down- 
wards. It is only in consequence of their being near together, so 
that the lines of spore-cases become confluent laterally, that they 
look at all like Péeris, with which they really have no structural 
affinity. They are, in fact, much nearer the Adiantew, only there 
the receptacles are resupinate, that is, on the reflexed indusiate 
portion, and therefore in reality turned upside down. 

