REPRODUCTIVE ORGANS OF PLANTS. 295 
volvulus. The cut represents a magnified section length- 
wise through the short pistil; a, is the stigma or summit 
of the pistil; 6, are grains of pollen; c, are pollen tubes 
that have penetrated into the seed- 
vessel which forms the base of the 
pistil; one has entered the mouth of 
the rudimentary seed, g, and reached 
the embryo sack,.e, within which it 
causes the development of a germ; d, 
represents the interior wall of the 
seed-vessel; A, the base of the. seed 
and its attachment to the seed-vessel. 
Darwin has sbown that certain 
plants, which have pistils and stamens 
in the same flower, are incapable of 
self-fertilization, and depend upon in- 
sects to carry pollen to their stigmas. 
Such are many Orchids. 
Artificial Fecundation has been 
proposed by Hooibrenk, in Belgium, 
as a means of increasing the yield of certain crops. Hooi- 
brenk’s plan of agitating the heads of grain at the time 
when the pollen is ripe, in order to ensure its distribution, 
which is done by two men traversing the field carrying a 
rope between them so as to lightly brush over the heads, 
appears to have been found very useful in some cases, 
though in many trials no good effects have followed its 
application. We must therefore conclude that agitation 
by the winds and the good offices of insects commonly 
render artificial assistance in the fecundating process en- 
tirely superfluous. 
Hybridizing.—As the union of the sexes of different 
kinds of animals sometimes results in the birth of a hybrid, 
so among plants, the ovules of one kind may be fertilized 
by the pollen of another, and the seed thus developed, in 
its growth, produces a hybrid plant. In both the animal 
