41 8 ELEMENTARY BIOLOGY. [CHAP. 



are developed from them by outgrowths of their walls ; and 

 this process is repeated by the branches, until the hyphaa 

 proceeding from a single conidium may cover a wide circular 

 area, as a patch of mycelium. When, as is usually the case, 

 many conidia germinate close together, their hypha? cross 

 one another, interlace, and give rise to a papyraceous crust. 

 After the hypha? have attained a certain length, the pro- 

 toplasm divides at intervals, and transverse septa are formed 

 between the masses thus divided off from one another. But 

 neither in this, nor in any other Fungus, are septa formed in 

 the direction of the length of the hypha. 



Very early in the course of the development of the 

 mycelium, branches of the hypha? extend downwards into 

 the medium on which the mycelium grows ; while, as soon 

 as the patch has attained a certain size, the hyphse in its 

 centre give off vertical aerial branches, and the develop- 

 ment of these goes on, extending from the centre to the 

 periphery. The outgrowth of pencil-like bunches of branches 

 at the end of these takes place in the same order; and 

 these branches, becoming transversely constricted as fast as 

 they are formed, break up into conidia, which are ready to 

 go through the same course of development. 



The conidia may be kept for a very long time in the dry 

 state, without their readiness to germinate being in any way 

 impaired, and their extreme minuteness and levity enable 

 them to be dispersed and carried about by the slightest 

 currents of air. The persistence of their vitality is subject 

 to nearly the same conditions of temperature as that of 

 yeast. Pullulating cells, resembling Torula;, are not un- 

 frequently derived from the conidia of Penicillium and many 

 other of the lower Fungi, but they must not be confounded 

 with true Yeast. 



In Penicillium we have for the first time a differentiation 



