824 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



host plant. These hyphal threads bifurcate, and the end of each branch swells up 

 into a spore. The supporting hyphal branch then grows out again below each 

 spore, elongates, and extends upwards, and pushes the spore on one side. The 

 result of this oft-repeated process is a structure which resembles a small much- 

 branched tree, with egg-shaped fruits hanging from the boughs. The hj'phal 

 branches, on which the spores are set like fruits, are cylindrical, stiff, and tiu-gid in 

 damp air, but in dry air, especially when they are ripening, they become ribbon-like 

 and spirally twisted so as to resemble cotton-cells. They are extremely hygroscopic, 

 and the slightest change in the humidity of the surrounding air is enough to 

 increase or diminish their spiral torsion. Even mere breathing on them produces 

 an alteration in the twisting, and if a rapid and marked alteration occurs in the 

 hygroscopic condition of the environment, the branches with their hanging spores 

 are whirled hither and thither, and the spores, which are only attached but slightly, 

 are scattered in all directions. This cannot of course be seen except under unusually 

 favourable circumstances, on account of the minuteness of the spores. 



The shedding of the spores can be observed with the naked eye in the Mould 

 Pilobolus cristallinus, one of the Mucorinese, shown in figs. 456 ^ and 456 ". The 

 mycelium of this Mould consists of a colourless, much-branched tube, and grows on 

 the excrement of horses and other mammals. Enlargements arise on the mycelium, 

 and from each is produced a sporangial mechanism composed of two parts, a colour- 

 less, barrel-shaped, stalk-cell and a dark head. The latter contains a colourless 

 jelly, which swells up in water, together with numerous spores, and is to be regarded 

 as a sporangium. Its wall is covered with calcium oxalate, so that its elasticity is 

 completeljr lost and it becomes brittle. The cell-wall of the barrel-like swollen 

 stalk, however, remains soft and elastic. At the junction of the dark sporangium 

 with its colourless stalk a circular layer of separation is formed. When the 

 turgidity of the sporophore increases in consequence of the absorption of water 

 from the mycelium the tension at last becomes so great that it causes a rupture 

 round the circular line mentioned. At the same moment, however, the elastic wall 

 of the part of the sporophore immediately below contracts, and the fluid contents 

 are pushed out with great force. The push is transmitted to the dark sporangium 

 above the split, and both the fluid contents of the club-shaped support and the 

 entire sporangium are thrown off" (see fig. 456 ^). The force of the explosion is so 

 considerable that the dark mass is raised about a metre in height. The whole 

 process, which, as we have said, may be seen with the naked eye, usually occupies 

 18-20 hours. The development of the mechanism begins at mid-day; during the 

 night the spores are formed in the vesicle, and the next morning the explosion 

 occurs as soon as daylight appears. 



A no less interesting spectacle is afforded by the scattering of the unicellular off- 

 shoots, i.e. conidia, in species of the genera Empusa and Entomophthora. These live 

 on the dead bodies of caterpillars, flies, aphides, and other insects, the commonest 

 and best known being Etnpusa muscoi, which lives on the common house-fly. When 

 a conidium of this Empxisa falls on the body of the fly it puts out a tube which pene- 



