144 BOTANY. 



256. The mycelium sometimes develops exclusively in 

 the interior of the nutrient medium ; in other cases it de- 

 velops partly in the medium and partly in the air. In 

 some species the mycelium may occasionally attach itself 

 to the hyphae of other plants of the same family, and even 



Fig. 74.— Diagram showing the mode of growth of Mucor mucedo. m, 

 the mycelium ; s, single spore-case, borne on an aerial erect hypha. 



to nearly related species, and derive nourishment parasiti- 

 cally from them. It is doubtful, however, whether any 

 species are entirely parasitic, and so far as parasitism occurs 

 it appears to be confined to narrow limits ; none, so far as 

 known, are parasitic upon higher plants. 



257. The reproduction of black moulds is asexual and 

 sexual. In the asexual reproduction the mycelium sends 

 up erect hyphse (Fig. 74), which produce fewer many sepa- 

 rable reproductive cells — the spores. The method of for- 

 mation of the spores in a common black mould (Mucor 

 mucedo) is as follows: The vertical hyphse, which are 

 filled with protoplasm, become enlarged at the top, and in 

 each a transverse partition forms {A, a, Fig. 75), the por- 

 tion above the partition (5) becomes larger, and, at the same 

 time, the transverse partition arches up {B, a), finally ap- 



