INTRODUCTION. 
7 
“ The sorus is in general situated on a receptacle, which, when roundish, consists 
entirely of short spiral vessels, so called, vermicoid bodies, similar to the thickened 
extremities of the leaf-nerves, which might therefore be regarded as abortive 
receptacles ( query thecae). In the elongated receptacles, spiral vessels are also 
met with.” The sori are in some tribes of the Ferns naked, but in the generality 
covered with a scale or indusium, of shape similar to themselves. I remarked in 
the first edition, that the origin of this integument was undoubtedly similar to 
that of the scales, namely, disrupted epidermis. Microscopic observations, 
however, induce me to doubt this assigned origin for the indusia, and to consider 
them as distinct organs, as much so, indeed, as the calyx of a flowering plant, or 
the calyptra of a Moss. Whether they arise from the vermicoid bodies of Link, 
just spoken of, or not, I have been prevented from observing. These certainly 
attend the genus Woodsia, and are intermingled with the thecae, the genus having 
no real indusium. In the genera Cystopteris, of Aspidium Adiantum, Pteris, 
&c., the reticulation of the indusium and epidermis is very different, and the 
former is not furnished with stomata. In many genera this organ may be sup- 
posed a part of the frond itself turned over upon the thecae, as in Adiantum and 
Pteris, but I believe the reticulation is very distinct. 
NUMBER AND DISTRIBUTION. — For the following remarks upon this 
subject I am indebted to II. C. Watson, Esq., than whom no botanist has more 
studied the subject. He says, “ Dorsiferous Ferns are found in every part of 
Britain, except on the summits of the loftier mountains, and in small spaces of 
the lower grounds, whence they are banished by local peculiarities of the soil or 
surface. But overlooking these merely local exceptions, of trifling extent, Poly- 
podiaceae may be stated to range over the whole of Britain, from south to north, 
from east to west, and from the shores of the sea almost to the summits of the 
highest hills ; in which latter situation their absence is to be attributed rather to 
the bleak exposure than to the absolute height. The number of our dorsiferous 
Ferns will be estimated variously, according to the views entertained with respect 
to union or division of reputed species, but 35 is the number most generally 
received. These bear a proportion to flowering plants (reckoning the latter 
at 1400), of 1 to 40. The order has a great numerical predominance over 
the other orders of Filicales, all taken together ; the proportion of its species 
being to those of the other three orders, as 6 to 1. And since the most 
abundant and widely-ranging species of Ferns are also to be found amongst the 
Polypodiacere, the effect of this order in the general vegetation of our island 
much exceeds that of the allied orders of Ferns. Of the six species referred to 
other orders, one is exclusively an inhabitant of Yorkshire (if it really be there 
still) ; namely, Trichomanes brevisetum ; a second, Hymenophyllum Tunbrigense, 
is local; a thud, H. Wilsoni, though much more plentiful, is limited to the 
northern and western counties ; whilst the other three, the Osmunda, Botrychium, 
and Ophioglossum, though widely scattered through Britain, are by no means so 
generally present as many of the Polypodiaceac. Several of the British dorsi- 
ferous Ferns are so widely and plentifully scattered throughout the island, that 
