REVIEWS. . 271 
and above water also are more or less covered or incrusted with 
these permanent forms of vegetation. We know also that they 
speedily appear on most recent erections of every kind, except 
these buildings be metallic. No botanist, so far as we are aware, 
has assigned any limits to the duration of this class of plants: 
the terms perennial, biennial, and annual do not occur to perplex 
the beginner with the uncertain periods they are employed to 
define. Lichens and Mosses also, we believe, baffle the chrono- 
loger of vegetation, and he prudently leaves the question, how 
long do they live? in all its original obscurity. 
The limits of the individuals composing this Order are almost 
as difficult to define as their duration is. At one end of the line, 
or series, or with some part of the circumference, they touch the 
order Hepatice (Liverworts) ; at another, they appear to frater- 
nize with the Fungi (Mushroom Family) ; and by a third, with 
the Alge (sea and fresh-water weeds). Notwithstanding this con- 
nection or affinity with nearly all the Cryptogamic Orders, the 
majority of them are readily distinguishable from the greater 
bulk of the individuals of these families. From the Liverworts 
the Lichens may be distinguished by their colour and their usu- 
ally firmer consistency : the former are green, or some modifica- 
tion of green ; the latter, it may be said, are rarely, if ever, green: 
* they are often olive, brown, scarlet, stone-colour, yellow, white, 
and almost all colours and shades of colour except green. They 
are generally of a dry, parchment-like, or leathery, or thread-like 
texture, if such a word is permissible. Some of them have defi- 
nite shapes, as the Fairy Cup-moss; but they are usually more 
or less shapeless extensions, variously lobed and divided. Some 
of them grow erect on the ground or on rotten rocks; others 
hang on the trunk or branches of trees; and by far the larger 
part of them spread out horizontally on the ground or on the 
rocks, ete. Their substance is cellular (composed of cells), or of 
cells with some fibres intermixed. They are without true roots 
(some of them have this organ in an imperfect state), or stems, 
or leaves. They are exceedingly simple, consisting of a thallus 
(either a flat expanded substance or a more or less branching flat 
and linear form), or a crust, and the fructification. The thallus 
or crust is often separable from the medium on which it grows, 
but it is also inseparable, and can only be detached by detaching 
a portion of the stone or bark or wood on which it grows. The 
