Botanical Society of Edinburgh. 353 



not usually arranged in a continuous 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 natter cells, forming an imperfect epidermal 

 covering. 



" The green tissue appears to represent the living and actively 

 vegetating part, of the lichen — determining by its development the 

 form of the frond, and giving origin to all the other tissues. The 

 cells appear to be in some degree independent 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 hvgrometric pro- 

 perties, absorbing water rapidly and thereby undergoing great change 

 of form. 



"This tissue is leplaced in many lichens by an unorganized colloid 

 matter, also hvgrometric to a great degree. It is sometimes nearly 

 absent, and under whatever form it appears, it seems to act mecha- 

 nically only, transmitting pabulum to the green layer, and keeping it 

 surrounded by a sufficient quantity of moisture. The green cells 

 termed gonidia frequently accumulate in masses, burst through the 

 cuticular layer, and appear as a green powder on the surface of the 

 plant. In this state the single gonidia are capable of continuing the 

 powers of cell-development at a distance from the parent — forming 

 round themselves the gray hvgrometric tissue ; and, like the parent 

 plant, producing at length true reproductive organs. This is by no 

 means a solitary instance of the formation of these, from developing 

 cells in the vegetable kingdom. "We have in the Ferns an instance of 

 another order propagating through gonidia. In the Ferns, cells, long 

 called spores, are found within modified leaves or parts of leaves. 

 These cells, when placed in favourable circumstances of heat and 

 moisture, develope, by nuclear division, a small cellular expansion 

 (still part of the parent plant, as no process of cell-conjugation has 

 intervened) called the pro-embryo. On this pro-embryo two cellules 

 of different character appear, a union takes place between the dif- 

 ferent cells, and the product is an ovoid body, the sporidium. Within 

 this sporidium, by nuclear division, spores are produced, only one of 

 which comes to perfection, the others proving abortive. The spore 

 is developed in situ, feeding upon the pro-embryo, as upon a cotyledon, 

 and forming the new fern. 



" To return to the Lichens. If we examine sections made through 

 the frond of Lecanora — through the apothecia at various stages of 

 growth, we meet at an early stage with a hollow sphere of delicate 

 rounded cells (perithecium), surrounding a number of elongated fili- 

 form cells (paraphyses) arranged vertically in a rounded mass. 



" Advancing a little farther in development, the cells of the peri- 

 thecium above the centre of the mass of paraphyses have given way, 

 and among the paraphyses a few flask-shaped, delicate cells (asci) are 

 visible, closely resembling paraphyses distended and filled with mucus 

 or cytoblastema. 



" Very shortly the fluid contents become slightly granulated, and 



