GROWTH OF MOULD ON DECAYING SUBSTANCES. 341 



other preserves; especially when they are viewed with a low 

 magnifying power, by reflected light. For they present them- 

 selves as a forest of stems and branches of extremely varied and 

 elegant forms (Fig. 124), loaded with fruit of singular delicacy of 

 conformation, all glistening brightly on a dark ground. In re- 

 moving a portion of the "mould" from the surface whereon it 

 grows, for the purpose of microscopic examination, it is desirable 

 to disturb it no more than can be helped, in order that it may be 

 seen as nearly as possible in its natural condition ; and it is there- 

 fore preferable to take up a portion of the membrane-like sub- 

 stance whereon it usually rests, which is, 

 in fact, a mycelium composed of interlacing 

 filaments of the vegetative part of the plant, 

 the stems and branches being its reproduc- 

 tive portion ( 213). The universality of the 

 appearance of these simple forms of Fungi 

 upon all spots favorable to their develop- 

 ment, has given rise to the belief that they 

 are spontaneously produced by decaying 

 substances; but there is no occasion for 

 this mode of accounting for it ; since the 

 extraordinary means adopted by Nature 

 for the production and diffusion of the 

 germs of these plants, adequately suffices 

 to explain the facts of the case. The num- 



T n i 1-1 T7i Stysanus caput-meclusce. 



her of sporules which any one Fungus 



may develope, is almost incalculable ; a single individual of the 

 puff-ball tribe has been computed to send forth no fewer than ten 

 millions. And their minuteness is such, that they are scattered 

 through the air in the condition of the finest possible dust ; so 

 that it is difficult to conceive of a place from which they should 

 be excluded. This mode of explanation has received further 

 confirmation from the facts recently ascertained, in regard to the 

 great number of forms under which a single germ may develope 

 itself; the particular form being determined, it seems likely, by 

 the soil whereon each germ happens to grow. Hence we are not 

 obliged to suppose that distinct gei-ms are floating about in the 

 atmosphere, for all the forms of fungous vegetation which appear 

 to be of different species, and which are only found in particular 

 situations, the Puccinia rosde, for example, only upon rose- 

 bushes, Isaria felina only upon the dung of cats deposited in 

 humid and obscure situations, and Onygena exigua upon the hoofs 

 of dead horses ; but are warranted in believing that the real va- 

 riety of germs is comparatively small, and that the facts just 

 stated, with others of the same order, only indicate the modify- 

 ing influence of the circumstances under which they are de- 

 veloped. 



212. The parasitic Fungi which infest some of the Vegetables 

 most important to Man, as furnishing his staple articles of food, 



