OF FILAMENTOUS, FRUTICULOSE, AND FOLIACEOUS LICHENS. 115 
with age. From its density, the nucleus can be frequently readily separated 
from its capsule and from the thallus by the point of a needle, as in many Aica- 
solias and Stictas, where it is comparatively large. Its colour is usually whitish 
or grayish ; sometimes it is rose-coloured ; seldom is it so different from that of 
the surrounding medullary tissue as to be readily distinguished therefrom. At other 
times, however, this difference or contrast in colour between the nucleus and the 
surrounding medullary tissues is the only means of distinguishing the former on 
section of the thallus. In old age, the nucleus frequently falls out spontaneously, 
leaving a cavity, which is at first not very conspicuous, but which gradually be- 
comes so by being deepened, acquiring a dark colour, and having swollen lips or 
margins. These cavities, which are sometimes very irregular in form, frequently 
give the thallus the appearance of being covered over with perforations, more or 
less closely aggregated, as in Parmelia saxatilis, var. omphalodes, P. encausta, and 
P. stygia. - 
a. Sterigmata.*—These are delicate filaments, arranged vertically to the 
wall of the spermogone, and convergently to its central cavity. They consist 
either of a single elongated cell, or its ramifications; or of a number of shorter, 
and usually broader cells in superposition. Hence they are divided into simple, 
and compound or articulated | Arthro-sterigmata, Nyl.|| They are so closely ag- 
gregated, that, under a low power, they look like mere striz; and sometimes, 
under the microscope, they appear united at their bases. Their walls differ much 
in thickness ; they are usually thickest in some arthrosterigmata. They appear to 
contain a colourless, homogeneous fluid; and their walls are also colourless, 
generally speaking, though occasionally in age their bases acquire a certain degree 
of colour. The tissue which they form is very hygrometric, imbibing water with 
ereat rapidity and ease. 
(1.) Form a. Simple, filiform, and 1-cellular, or consisting of the ramifications 
of a single cell. 
Longish in Ramalina, Lichina, Roccella, Spherophoron, Stereocaulon, Alectoria, 
and Dufourea. 
Shortish, sometimes almost absent, in Sguamaria. Simple sterigmata some- 
times taper gradually into spermatia, which then appear as terminal joints. 
They generate spermatia only from their apices. They divide or branch at or 
near the base only, and their ramifications sometimes resemble digitate processes 
from basal tubes or filaments. In the old state, the simple sterigma becomes 
sometimes sterile, elongated, and ramose; and its ramifications form a filamen- 
tous network more or less occupying the spermogonal cavity, and resembling that 
which is found in the spermogone of many Parmelias having articulated sterig- 
mata. This is particularly noticeable in some Cladonias. 
* origrywo, a support, in allusion to their function of generating the Spermatia. 
