THALLOGENS. 269 
in the Usneas; crustaceous, as in the Squamaria ; hypophleod, 
or hidden under the epidermis of the trees, or under the woody 
fibres, as in the Verrucaria, Xylographa, &e. 
In order to give some idea of the anatomical structure of the 
_ thallus, it will suffice to mention Parmelia parietina, the thallus of 
which does not exceed the three-hundredth part of an inch in 
thickness. 
Thin as it is, however, this organ presents four very distinct 
regions. Its upper part consists of a bed of thick closely consolidated 
cells, of a yellowish colour at their surface only. In front of the 
lower part of the thallus is another cellular bed, white, like the 
first. Between these two epiderms are confined, Ist, the green 
seeds known as gonidia, and formed of a bed called gonidial ; 
2nd, a kind of medulla, or pith, formed of filamentous elements 
loosely interlaced or knitted, which is the medullary bed, and 
Which encloses the air in the mesh. 
If we pass from the vegetative to the reproductive system, 
we find that it consists of the fructifying or female organs, and 
a fecundating or male apparatus; the first being represented 
by the Apothecceia—namely, the cups or shields containing the 
fructification ; the second by the Spermogonia. 
The apothecceia of fruits of the Lichens develop themselves on 
the upper face of the thallus, or upon that part of it which is turned 
towards the light. They strongly resemble the small disk or 
nucleus—black, brown, yellow, rose-coloured, red, and sometimes 
interspersed with brown or grey. In size they are extremely 
Variable ; the smallest are under the one-twentieth of an inch, 
whilst the largest may be an inch. 
The Spermogonia are generally very small organs, rounded or 
oblong, lodged sometimes in particular tubercles, but more 
* frequently immerged in the superficial beds of the thallus. _ 
re Y reasons lead to the conclusion that the spermogonia are 
®male organs of the Lichens. At first they present themselves 
Parallel in the fruit, or simultaneously with it in the same individual, 
at other times only upon sterile individuals in such a manner 
th . Mm the latter case the apothecceia and the spermogonia develop 
“mselves upon different individuals. The tenuity of the cor- 
Contained in the spermogonia, their immense numbers 
