1887] The Milkweeds. 609 
it will be left alone just as long as there is anything else to'be 
found. Should there come a favorable season, with an abundant 
vegetation and good grazing, such plants will then have an ex- 
cellent chance of perfecting seed. Even during a season when 
. plant-life must struggle for water, and when herbivorous animals 
greedily devour everything they can find, the milky-juiced plant 
will succeed better than any other, because it will be the last one 
to be touched. Thus from generation to generation the devel- 
opment will go on, until it reaches a state which requires no 
further change, and it then remains quiescent. Having, how- 
ever, Once acquired a feature, it will be a long time, indeed, 
before it will lose it. In the present instance the flowers seem 
to have been developing in their way, while the juice pursued its 
own. 
The flowers of the common species are produced in large 
clusters, are pink in color, and have quite a perceptible odor. 
They are visited by insects in great numbers, as many as thirty- 
one different species having been counted; and this fact shows 
they possess considerable attractive qualities. If we examine 
an individual flower of a cluster, a peculiarly complicated mech- 
anism will be revealed. e two floral en- 
velopes, calyx and corolla, are folded back or 
reflexed on the stem. Inside of these is a 
column made up of five sac-like bodies, each 
FIG. 2.—-Sac in 
9, which ; 
_ Fic. 1.—Flower of pollinium is Fic. 3.—Hood, 
Asclepias enlarged. Steed: with horn. 
_ crowned by a horn-like process. These seem to be nectaries, 
where honey is secreted. Between the nectaries there is found, — 
on examination, a peculiar slit, wider below than above. Atthe 
_ upper end is a hard, thin lamina, or blade, with the sides bent _ 
_ inwards so that the edges are close together. In swollen sacs 
