370 Dr. W. Ny lander on the Gonimic 



XLV. — NotulcB LichenologiccB. No. XXIV. 

 By the Rev. W. A. Leighton, B.A., F.L.S. 



On the Gonimic Evolution of the Collemacei'^ . 

 By Dr. W. Nylander. 



The nature of tlie Nostocs has been frequently investigated ; 

 and an hypothesis has prevailed that a Nostoc is 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), isgranulose 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 

 liSiiaYQ pichneum of a variety of Leptogium tremelloides in Ach. 

 Syn. p. 343). This, in like manner, afifords 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 papillae 

 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. 



