INTRODUCTION TO CllYPTOGAMIG BOTANY. 75 



no difficulty in admitting the vulgar notion, that Poly- 

 yorus squamosus is a mere product of the sap of the tree 

 on which it grows, or even that plants and animals, how- 

 ever complicated, may spring from decomposed or decom- 

 posing organic matter ; especially if the notion be extended 

 still farther, as it is by many German authors, so as to include 

 the possibility of this same protoplasm (urschleim) being 

 equally capable of giving rise to the lower animals. The 

 ground, however, is fast sliding from beneath the feet of such 

 philosophers ; organisms which once were supposed to be so 

 simple, are found to be, in fact, somewhat complicated ; the 

 presumed distinctions of animal and vegetable life are not 

 so certain as was imagined by older physiologists ; and in 

 consequence, when a Protococcus (Fig. 8, 9), in one stage 

 of growth produces veritable spores, and in another bodies 

 endowed with apparently voluntary motion, by means of 

 flagelliform appendages, we are not at once to presume that 

 such effects take place indifferently, but should conclude, rather, 

 that they are bound up in the very nature of the production, 

 and that the two kinds of bodies are no more organisms gene- 

 rically or specifically distinct from the parent, than the male 

 offspring is from the female in the animal kingdom. 



58. Thallogens consist, in most cases, of cells, modified in 

 form, and in the nature and chemical condition of their walls, 

 but never accompanied by spiral tissue, except in the organs of 

 fructification, where its office seems to be principally the gradual 

 dispersion of the reproductive bodies. Whatever aeration may 

 take place in their substance, it is at least not conducted by spiral 

 vessels or by analogous ducts, but by large intercellular passages, 

 or the void space between the component filaments, where the 

 cells, as is very frequently the case, are drawn out into threads. 

 They are entirely destitute of true stomates, though in many 

 cases, from their spongy nature, the air has free access into 

 their substance. In many, however, the inner cells must be 

 cut off from communication with the atmosphere, except by 

 eudosmose, and in the aquatic species, external apertures are 

 scarcely to be expected. Cladonia retiiwra (Fig. J 9) presents a 

 frond remarkable for its perforation, and some other Lichens in 



