294 ON THE PROGRESSIVE DEVELOPMENT OF 



the varied tribes of this part of Nature's wide domain, is so 

 gradual, and the relation by which many of these tribes are 

 connected together, so intimate, that the difficulty which 

 botanists experience in the arrangement of the objects of their 

 studies frequently consists more in the separation of allied plants 

 and families than in their union with each other into groups. 



The structure of the powdery lichen, to which allusion has 

 already been made, and of its allied species, is most simple, con- 

 sisting of vesicles only, and those scarcely connected together. 

 Other forms of the vast tribe of lichens — those humble individuals 

 of the vegetable race which form the first of Nature's coverings — > 

 which act, as it were, the part of pioneers, and though of small 

 repute, and to the unobservant eye insignificant, are the very 

 instruments which it has pleased the Almighty Creator and Ruler 

 of all to employ as the secondary agents of his power in breaking 

 down the surface of the hard and barren rock, and rendering 

 many a wild and uncultivated region fit for the nourishment of 

 larger tribes of plants, and ultimately adapted to the support of 

 the higher orders of animals, yes, even of man himself. — Other 

 forms of this vast, and shall we not say important family of 

 plants, are found, in which these primary vesicles are closely 

 compacted into crusts or films, — crusts of extreme tenuity, but 

 covering the surface of stones and rocks, and the bark of trees. 

 Such are the curious Lecidea geographica, so called from its 

 resemblance to a chart or map ; the Graphis elegans, and Ope- 

 grapha scripta, a species of mimic writing, &c. The transition 

 from these filmy expansions to the foliaceous lichens, such as the 

 common yellow wall lichen, Parmelia parietina, and its allies, by 

 the splitting up, as it were, of the crust in different directions 

 from the surface to which it is attached into scales or lamina of 

 various thickness, as in Squamaria, and others of the scaly lichens, 

 is well marked; and again from these through the Cetraria 

 Islandicus, or Iceland moss, and others in which these scaly and 

 leafy crusts are gradually lengthened into thongs, to the fibrous 

 or filamentous lichens on the one hand, and into some, of the 

 tribe of JlgcB or sea weeds on the other. 



The same gradual development of the cellular structure may 

 be traced in the Algcs, a family of plants, the greater number of 

 which are inhabitants of the ocean. This we have already had 

 occasion to observe in their fructification from the simple vesicles 

 of Porphyra and Ulva, to the variously compounded granules of 

 Gracilaria, Rhodomela, and Microcladia, changes accompanied by 

 corresponding variations in the structure and configuration of 

 the entire plants. 



The transition from the Algce to the mosses, in which the 

 external form approaches still more to those vegetable tribes 

 where the cells, becoming gradually more elongated, approach 

 more or less to the tubular structure, is well marked by some of 

 the Hepatici, or liverworts. Of these last, the frond or leafy 



