3* ELEMENTARY BIOLOGY. [CHAP. 



formed by the wall, and present large central clear spaces, 

 or vacuoles. At intervals, transverse partitions, continuous 

 with the walls of the tube, divide it into elongated cells, 

 each of which contains a . correspondingly elongated proto- 

 plasmic sac, or primordial titride. The hyphae frequently 

 branch dichotomously ; and, in the crust, they are inex- 

 tricably entangled with one another ; but every hypha, with 

 its branches, is quite distinct from every other. Those 

 aerial hyphse which are nearest the periphery of the crust 

 end in simple rounded extremities ; but the others terminate 

 in brushes of short branches, and each of these branches, as 

 it grows and elongates, becomes divided by transverse con- 

 strictions into a series of rounded spores arranged like a 

 row of beads. The spores formed in this manner are 

 termed conidia. At the free end of each filament of the 

 brush the conidia become very loosely adherent, and con- 

 stitute the green powdery matter to which reference has 

 been made. Examined separately, a conidium is seen to be 

 a spherical body, composed of a transparent sac, enclosing 

 a minute mass of protoplasm, in all essential respects similar 

 to a Tonda. If sown in an appropriate medium, as for 

 example Pasteur's solution, with or without sugar, the coni- 

 dium germinates. Upon from one to four points of its 

 surface an elevation or bulging of the cell-wall and of its 

 contained protoplasm appears. This rapidly increases in 

 length, and, continually growing at its free end, gives rise to 

 a hypha, so that the young Penicillium assumes the form of 

 a star, each ray being a hypha. The hyphas elongate, while 

 side branches are developed from them by outgrowths of 

 their walls ; and this process is repeated by the branches, 

 until the hyphae 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, 



