THE DISPERSAL OF SPORES BY THE WIND 559 



the same as tliat of violent basidiosjjore-discharge, i.e. violent dis- 

 charge is a means of sejjarating the moist adhesive spores from 

 their moist and living substratum and ejecting them into the air 

 in such a way that they may be carried off by the wind and thus 

 aid in the dispersal of the species. The discharge of aecidiospores 

 to a distance, varying with the species, of from 4 to 15 mm. is 

 violent enough to eject even the last-formed spores at the bottom 

 of the aecidia into the air well beyond the rim of the peridium. 

 The opening out of the peridium, its radial sj^litting into rays, and 

 its becoming more or less revolute make for efficiency in the liber- 

 ation of the spores ; for, by functioning in this way, the peridium 

 forms as small an obstacle as possible to the escape of the spores 

 as they are shot outwards from the interior of the aecidium. 



The peridium is composed of living cells which fit together to 

 form a continuous sheet of cells one cell in thickness. In some 

 species, e.g. Puccinia Caricis (aecidia on Nettle), on the outside of 

 the peridium, the wall is much thicker than on the inside and a 

 piece of the cell-wall of one cell stretches downwards and overlaps 

 the upper part of the cell-wall of the next lower cell, so that between 

 two successive peridial cells there is a kind of hinge. This dis- 

 position of the cell-walls is doubtless of functional significance. As 

 the peridial cells increase in length in a direction parallel to that of 

 the axis of the aecidium, their thicker outer walls stretch less than 

 their thinner inner walls and, at the same time, the outer thicker 

 wall of the peridium as a whole tends to preserve its continuity, 

 while bending at its hinges, whereas its inner wall tends to lose its 

 continuity owing to the cells rounding themselves off. Hence, as 

 growth takes place, the j^eridium readily bursts, opens out, splits 

 radially into rays, and becomes revolute. To what extent, if any, 

 the lower part of the peridium, which encloses and presses against 

 the tightly packed chains of spores contained within the aecidium, 

 assists in providing the force employed in the discharge of the spores 

 remains to be investigated experimentally. 



The Dispersal of the Spores by the Wind. — A uredinous 

 basidiospore which has been shot away from its sterigma into the 

 air is carried off by the wind in the same manner as the spores 

 of the Hymenomycetes. Owing to its small mass, its relatively 



