112 Elementary Biology. 



We have to note in the first place the comparative 

 importance of the thallus or sexual generation, which in 

 Polytrichum assumes the external appearance of, and simu- 

 lates in internal structure, an ordinary flowering plant We 

 note further the existence of an asexual generation which 

 is parasitic on the sexual generation, although the succes- 

 sion of asexual generations in Penicillium is here represented 

 by the power on the part of the sexual thalli to separate off 

 portions of themselves in the form of protonemata, which 

 are capable of producing again a sexual thallus by ordinary 

 vegetative division. We note, moreover, that the spore- 

 producing generation is a highly organised structure, and 

 that it carries distinctively a sporangium, some of the con- 

 tained cells of which form an archesporium, i.e. a group or 

 layer of cells capable indirectly of producing spores or 

 asexual cells, which, without union with any other cell, are 

 able to form protonemata and new sexual plants. The seta 

 and theca of the moss correspond, therefore, to the stalk 

 and spore-bearing head of the fungus, together with that 

 part of the mycelium which bears the sporangium. We 

 have in the moss, in short, a plant which has made the 

 most possible of its sexual stage, while the asexual plant, by 

 being parasitic on the sexual, has arrested that development, 

 and laid the foundation of a new type of plant altogether, 

 viz. the asexual spore-bearing generation foreshadowed in 

 Penicillium. 



