407 



many lichens by an unorganized colloid matter, also hygronietric to a 

 great degree. It is sometimes nearly absent ; and, under whatever 

 form it appears, it seems to act mechanically only, transmitting pabu- 

 lum to the green layer, and keeping it surrounded by a sufficient quan- 

 tity of moisture. The green cells termed gonidia frequently accu- 

 mulate in masses, burst through the cuticular layer, and appear as a 

 green powder on the surface of the planl. In this state the single 

 gonidia are capable of continuing the powers of cell-development at 

 a distance from the parent, fonning round themselves the gray hygro- 

 metric 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 different cells, and the product is an 

 ovoid body, the sporidium. Within this sporidium, by nuclear divi- 

 sion, spores are produced, only one of which comes to perfection, the 

 others proving abortive. The spore is developed in silu, 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 

 ofLecanora, 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, filiform cells 

 (paraphyses), arranged vertically in a rounded mass. Advancing a 

 little further in development, the cells of the perithecium above the 

 centre of the mass of paraphyses has given way ; and among the para- 

 physes a few flask-shaped, delicate cells (asci) are visible, closely re- 

 sembling paraphyses distended and filled with mucus or cytoblastema. 

 Very shortly the fluid contents become slightly granulated ; and the 

 granules eventually aggregate into eight cytoblasts. Round these cy- 

 toblasts delicate, rounded cells are formed, which take at length an 

 ovoid form ; and we may generally easily perceive within them two 

 free nuclei. Round these nuclei two secondary cells are developed, 

 which gradually increase in size, so as nearly to fill up the parent-cell. 

 They become filled with densely-granular chlorophyll ; and finally the 

 two cells conjugate ; that is to say, the walls of both cells give way, 

 VOL. IV. 3 s 



