399 
Gleichenia, and is far from being weakened by such thecse as those of Par- 
keria. In Ophioglossum another kind of pro\dsion is made for the production 
of sporules, which in that genus seem to have no theca whatever beyond the 
involute contracted segments of the frond 'which bears them. WTiat are 
called the thecse in Ophioglossum are improperly so termed, and are much 
more analogous to the involucre of Marsilea. 
The stems of Ferns, when arborescent, are objects of great interest to the 
botanist, partly on account of their rarity, secondly, because of their singular 
structure, and especially because they offer the highest form of developement 
in Flowerless Plants. It has not been till lately that they have been weU 
understood ; they have now, however, received full illustration from Mohl in 
Martins’s beautiful leones plantarum cryptogamicarum. 
Bory de St. Vincent elevates Ferns to the rank of a class intermediate between 
Monocotyledons and Acotyledons ; but at the same time he attaches no im- 
portance to the descriptions of those 'writers who, having seen the geniiination 
of the sporules, have attempted to prove an identity between them and Monoco- 
tyledons in that respect. He justly observes, that the irregular unilateral scale 
which has been seen to sprout forth upon the first commencement of their growth 
is extremely different from the cotyledon of Monocotyledons, which pre-exists 
in the seed and never quits it, but swells during germination, and acts as a 
reservoir of nutriment for the young plantlet. He most properly regards it 
as an imperfectly developed primordial leaf. Without, however, adopting this 
view, I think it may not be inexpedient to consider Ferns as a collection of 
genera equivalent to the alliances of Flowering Plants, and consequently divi- 
sible into different natural orders ; and accordingly that course has been 
adopted here. But as it would be inconvenient to separate the general ob- 
servations upon them, I have merely characterized the orders and placed the 
genera that belong to them under each. In doing this I have been most 
essentially aided by Dr. Hooker, who has kindly furnished me with a list of 
the genera belonging to the whole of what constitutes the Filices of Linnseus. 
Geography. The following proportions borne by Ferns to other plants 
in different latitudes wiU serve to give some idea of the manner in which they 
are geographically distributed. There is an enormous disproportion between 
Ferns and the rest of the Flora in certain tropical islands, such as Jamaica, 
where they are l-9th of the Phsenogamous plants; New Guinea, where 
D’Urville found them as 28 to 122 ; New Ireland, where they were as 13 to 
CO ; and in the Sandwich Islands, where they were as 40 to 160; and it is 
clear, from the collections of WaUich, that Ferns must form a most important 
feature in the Indian Archipelago. Upon continents, however, they are far 
less numerous ; thus, in equinoctial America Humboldt does not estimate 
them higher than l-36th; and in New Holland Brown finds them l-37th. 
They decrease in proportion towards either pole : so that in France they are 
only 1 -63d; in Portugal, 1-1 16th ; in the Greek Archipelago, 1 -227th; and in 
Egypt, 1-9 7 1st. Northwards of these countries their proportion again aug- 
ments, so that they form 1 - 3 1 st of the Pheenogamous vegetation of Scotland ; 
l-35th in Sweden; 1-1 8th in Iceland; 1-lO^th in Greenland; and l-7th at 
North Cape. (See a very good paper upon this subject by D’Urv'iUe, in the 
Ann. des Sc. Nat. 6. 51. ; also Browns Appendix to the Congo Voyage, 461). 
Bro'wn has observ>ed {Flinders, 584), that it is remarkable, that although 
arborescent Ferns are found at the southern extremity of Van Diemen’s Island, 
and even at Dusky Bay in New Zealand, in nearly 46° south latitude, yet they 
have in no case been found beyond the northern tropic. For an excellent 
account of the geographical distinctions of Tree Ferns see Martins leones 
plantarum cryptogamicarum, p. 81. 
Properties. The leaves generally contain a thick astringent mucilage, 
