XVI 
PREFACE, 
main frond by a raised membrane (costa ?) which is even 
continued sometimes to the base of the frond, and runs down 
on either side the stipes as a narrow wing. In some cases 
the bordering costa is not only raised but leafy and lobed, 
like the margin of the frond, and bears sori, such fronds 
thus becoming double edged. N.F., f. 609. Scol. vulg. margi- 
natum and description. The regular sori are much inter- 
rupted by this projecting membrane, as sometimes the veins 
are; sometimes the bordering laminae alone bear fruit, f. 11, 
while the central portion of the 
frond is frequently thickened by the 
redundant matter of the lamina, and 
shows a puckered, warted, muricated 
condition : N.F., f. 642, 597. The 
whole of the phenomena, observable 
in these varieties, suggesting that 
such fronds arise from a coinci- 
dent but imperfect development of 
three laminae in one. This ex- 
planation is, however, by no means 
conclusively affirmed ; because, in 
many such cases, the upper side of the frond gives no in- 
dication of the bordering except by a slight depression of its 
surface here and there, above the line of division — the veins 
continuing direct from the costa to the margin of the frond. 
It must be observed, however, that the varieties of Hart’s- 
Tongue which show sori only on the margin of the border- 
ing, N.F., f. 595, while the centre of the frond is destitute of 
them ; or which have sori more numerous than, or in 
lines not parallel with those in the centre, suggest a fun- 
damental distinctness of origin of the bordering parts, the 
marginal processes being occasionally as simple as the 
medial portion is loaded with warty and fluted excre- 
scences. 
6. The Fruit of this tribe presents also some marked 
variations both in position, structure, and function. As a 
