496 



sketch of the structure of lichens in general, and of their mode of 

 nutrition and reproduction. He commenced by giving a definition of 

 the terms used. He considered spores as being the ultimate germi- 

 nating cellules, the product of the division of the compound granular 

 cell which is the result of the union of the conjugating cells in cryp- 

 togamic plants ; sporidia as the compound granular cells, the product 

 of the union of conjugating cells ; proto-sporidia as the simple cells of 

 lichens in which the two conjugating cells are afterwards formed ; 

 gonidia as free cellules derived from and part of the cellular tissue of 

 the parent plant, capable of continuing to a certain extent their de- 

 velopment when free from the parent, without the intervention of the 

 true generative act of conjugation (the analogues of free buds or bul- 

 bils in Phanerogameae). Mr. Thomson also considered the pro- 

 embryo in feiTis and other cryptogams as the cellular expansion 

 formed by the development of the gonidium, and containing the con- 

 jugating cells. This pro-embryo, then, corresponds to the ordinary 

 cellular expansion of lichens. 



The author then examined the structure of Lecanora tartarea, a 

 crustaceous lichen holding a middle place between the foliaceous 

 and the pulverulent species : — " When we examine a section of the 

 frond of Lecanora tartarea, we meet, in the layer which immediately 

 adheres to the rock or bark, chosen for its place of growth, with a 

 mass of elongated, more or less filiform cells. Most of these cells are 

 empty ; some of them contain a slightly viscid fluid, and in a few 

 there appears to be an undeveloped nucleus ; the cells are delicate 

 and of a light gray colour. Resting immediately above these, and 

 sometimes struggling down among them, are groups of rounded cells 

 filled with bright coloured chlorophyll, not usually arranged in a con- 

 tinuous layer, but scattered in small irregular patches, or as isolated 

 cells among the gray tissue. Above the green cells we meet with 

 another layer of transparent tissue, closely resembling that below it. 

 In Lecanora, we have above all a layer of somewhat flatter cells, form- 

 ing an imperfect epidermal covering. The green tissue appears to 

 represent the living and actively vegetating part of the lichen, deter- 

 mining by its development the form of the frond, and giving origin to 

 all the other tissues. The cells appear to be in some degree inde- 

 pendent of one another, though showing an evident tendency to form 

 small aggregations. The gray tissue packs them in, and surrounds 

 them, but appears to undergo no further change in development. It 

 has powerful hygrometric properties, absorbing water rapidly, and 

 thereby undergoing great change of form. This tissue is replaced in 



