B70 Dr. W. Nylander on the Gonimic 
XLV.—Notule Lichenologice. No. XXIV. 
By the Rev. W. A. Lercuton, B.A., F.L.S. 
On the Gonimic Evolution of the Collemacet*. 
By Dr. W. Nylander. 
THE nature of the Nostocs has been frequently investigated ; 
and an hypothesis has prevailed that a Nostoc 1s a Collema in 
a young state, or that a Nostoc, on attaining a certain age, 
passes into a Collema. ‘The actual transition, however, has 
hitherto escaped observation. 
The real state of the case may be best demonstrated by 
attention to normal physiological conditions, rather than by 
observations founded on textures disrupted and torn asunder 
anatomically. 
In the genus Collema the thallus possesses no distinct cortical 
layer. An isidium, when present on the upper surface (where 
alone it occurs), is granulose or furfuraceous, black, aggregate, 
and more frequently marginal. These granules, which are 
manifestly the propagula of the lichen which extrudes them, 
show very clearly under the microscope the entire history of 
the evolution of a thallus, from its first origin from a cellule 
containing a single gonimium to a minute true Nostoc (at 
least there is no apparent difference), and ultimately to the 
perfect texture of a Collema. We can thus see the mode 
in which these granules increase in magnitude. They are 
sometimes globose, and sometimes form deformed nodules, 
at first very minute and afterwards gradually larger. Thus 
we have before our eyes copious examples of all the pri- 
mary stages. This evolution might be easily explained by 
means of figures, but it is also sufficiently intelligible without 
them. 
In the genus Leptogium, the thallus has a distinct cel- 
lular cortical layer. A crowded, papillose, or at length dac- 
tyloid isidium is observable on the upper surface (hence the 
name pichneum of a variety of Leptogium tremelloides in Ach. 
Syn. p. 343). This, in like manner, affords conspicuous ex- 
amples of the initiatory thalline evolution by means of such 
gemmules. And in this genus isidioferous thalli occur more 
frequently than in the genus Collema. These isidiose papilla 
or gemmules are very laxly fixed on the thallus, and are 
readily separated by a slight touch. Here, however, the stages 
of the evolution are less simple, by reason of the externally 
cellulose texture of the thallus. 
It is also worthy of observation that isidioferous thalli 
* Translated from ‘Flora,’ Sept. 10, 1868. 
