1883. ] Means of Plant Dispersion. 817 
In the willows and poplars the seed is enveloped in cottony 
fibers, as in the cotton plant, rendering it very buoyant. The air 
around a cottonwood tree (Populus monilifera) is well filled with 
these whitened objects, like little bunches of cotton, on a windy 
day in spring or early summer, when the pods open, thus verify- 
ing its popular name. 
In scattering the spores of cryptogams currents of air act ina 
most effectual manner. The spores of the vascular cryptogams, 
and of mosses, lichens and fungi, as fine as dust, can be carried 
by the winds as readily as dust, and lodged in any soil suitable 
for growth. In this respect they are like the pollen of wind-fer- 
tilized flowers. Since many of these plants are parasites, and in- 
jurious to animal and vegetable life, as well as beneficial in some 
cases in keeping in check forms even more destructive, their 
mode of diffusion offers a field of investigation of the greatest 
practical importance. If the germ theory of disease be true, the 
physician can find in the air one of the most efficient instruments 
for spreading disease. 
Many small seeds that have no special contrivance to secure 
their dispersion are blown away from the parent plant. Their 
intrinsic lightness is an adequate adaptation, as in the, case of 
spores, The only peculiarity may be in the pod itself, in its mode 
of dehiscence, or way of exposing the seed. It may open so 
that the wind has free access to the seed; or the seed may be 
caught in the act of falling and turned from its course. In the 
mustard family the valves of the pods sometimes, as in Arabis, 
loosen at the base and turn upward, thus exposing the seeds as 
they hang in the partition between the valves. A loculicidal de- 
hiscence or splitting of the wall of a capsule so as to open 
directly into a cell, favors this result. In the pink family the pod 
Splits up into several parts, often ten, effectually exposing the 
small seeds. As seeds thus to be dispersed by wind agency, may 
be mentioned those of figworts (Scrofulariacez), primroses (Prim- 
ulaceæ), poppies (Papaveracee) and gentians (Gentianacez), a list 
_ that might be much extended. 
Nor should we limit the power of the wind in spreading fruits 
and seeds to the lighter kinds, for it has an important effect on the 
larger and heavier. As they mostly ripen at the time of the year 
when the winds are usually strongest and often shifting in direc- 
tion, nuts and heavy fruits are blown to a considerable distance 
