422 ELEMENTARY BIOLOGY. [CHAP. 



proceed from the spore as a centre, their development 

 gives rise, as in Penicillium, to a delicate stellate mycelium. 

 At first, no septa are developed in the hyphas, so that the 

 whole mycelium may be regarded as a single cell with 

 long and ramified processes, or as an organism which does 

 not show a cellular structure at all. The distinction be- 

 tween this condition and the multicellular structure of 

 Penicillium is not so sharp as it appears at first sight, for 

 it has been shown that even in plants which consist of 

 many cells the protoplasm is very generally continuous, 

 through minute perforations in their walls. From near 

 the centre of the mycelium a branch is given off from a 

 hypha, rises vertically, and after attaining a certain length 

 ceases to elongate. Its free end dilates into a rounded 

 head, which gradually increases in size, until it attains 

 the dimensions of a full-grown sporangium; and, at the 

 same time, the protoplasm contained in this head be- 

 comes separated from that in the stalk by a septum, 

 which is curved towards the cavity of the sporangium, and 

 constitutes the columella. The wall of the sporangium, 

 thus formed, becomes covered externally with a coat of 

 oxalate of lime spines. As the sporangium increases in 

 size, its protoplasmic contents become marked out into a 

 large number of small oval masses, which are close together, 

 but not in actual contact. Each of these masses next 

 becomes completely separate from the rest, surrounds itself 

 with a cellulose coat, and becomes a spore; while the 

 protoplasm not thus used up in the formation of spores, 

 appears to give rise to the gelatinous intermediate sub- 

 stance, which swells up in water, referred to above. The 

 walls of the spores become coloured, and that of the spo- 

 rangium gradually thins, until it is reduced to little more 

 than the outer crust of oxalate of lime. The sporangium 



