190 PENICILLIUM AND AGARICUS LKsS. 



equal to about six or eight times its own diameter, the pro- 

 toplasm in it divides transversely and a cellulose septum is 

 formed (D, E, sef) dividing the young hypha into two cells 

 '(compare Fig. 36, H, p. 159). The distal cell then elongates 

 and divides again, and in this way the hyphae are, almost from 

 the first, divided into cells of approximately equal length. 



The mode of growth of the distal or apical cell of a hypha 

 is probably as follows. The free end tapers slightly (E) and 

 the cellulose wall thins out as it approaches the apex. The 

 protoplasm performing constructive more rapidly than de- 

 structive metabolism increases in volume, and its tendency is 

 to grow in all directions : as, however, the cellulose mem- 

 brane surrounding it is thinner at the apex than elsewhere, 

 it naturally, on the principle of least resistance, extends in 

 that direction, thus increasing the length of the cell without 

 adding to its thickness. Thus the growth of a hypha of 

 Penicillium is apical^ i.e. takes place only at the distal end, the 

 cells once formed ceasing to grow. Thus also the oldest cells 

 are those nearest the original spore from which the hypha 

 sprang, the youngest those furthest removed from it. 



A process which has been described as sexual, sometimes, but appa- 

 rently very rarely, occurs in Penicillium, and is said to consist essentially 

 in the conjugation of two gametes having the form of twisted hypha?, 

 and the subsequent development of spores in the resulting branched 

 zygote. But as the details of the process are complicated and its sexual 

 character is doubtful, it is considered best to do no more than call 

 attention to it. The student is referred to Brefeld's original account of 

 the process in the Quarterly Journal of Microscopical Science, vol. xv., 

 p. 342. The so-called sexual reproduction of the closely-allied Eitrotiuin 

 is described in Huxley and Martin's Elementary Biology (new edition), 

 p. 419, and figured in Howes's Atlas of Elementary Biology, pi. xix., 

 figs. xxvi. and xxvii. 



The nutrition of Penicillium is essentially like that of Mucor 

 (p. 167). But, as it has been remarked, "it is often content 



