r 
61 2 The Milkweeds. [July ; 
Orchid family. In this order the peculiarity has been more 
fully developed than in the first one, and the 
differentiation is greater. Ordinarily in the Or- — 
chids the caudicle, or stem, contracts and bends 
the pollinium forward; in the Asclepiads the i 
appendage twists in drying, and the two pollinia p 
are then brought close together. In both cases | 
the pollen-grains are only then in a position to © 
reach the stigma. Darwin has demonstrated that — 
the contraction of the caudicle of the Orchid pola 
linium is necessary to prevent its being put back 1 
Fic. 7.—-Pollin. into a cell similar to the one from which it was 
ium of Orchis. taken. In the Asclepiads the twisting of the ap- 
ndage brings the two pendent pollen-masses 
together, and enables them to slip into the slit between the hoods, — 
and be thus in a position to reach the stigma. 
The appendage to the seed of the Milkweed is of the same — 
‘something which is found in but one other family. This is the | 
ae 
character as that found in the Dandelion, the Thistle, and many a 
other plants. In all it serves the same end. Butin some species 
of the family the coma has the peculiarity of expanding very — 
rapidly on exposure to the air. In one of these (Gonolobus — 
obliquus) it is recorded by Mr. Meehan that the expansion of : 
the coma is so rapid as to be compared to a stroke of lightning. — 
He says it is followed by the eye with difficulty. While one © 
instant it is enclosed in the capsule closely packed away, as soon — 
as it is exposed the hairs of the coma fly round and form a per- 
to the parent plant, and the purpose of the development of the 
feathery appendage would be defeated; but, drying so rapidly, 
they are in a condition to be carried by the pica wind to 
great distances. 
_ fect hemisphere. This is a necessity, because if it did not so — 
occur, the seeds, being heavy, would fall to the ground close 
While in the common Milkweed the milky juice of the stem 
is so abundantly developed, it is a little remarkable’ that in an- 
other species (the Pleurisy Root) it is entirely wanting. The- 
flowers are similar, and the sap is so acrid as to cause the plant 
to be rejected by all animals. It grows in very poor soil, and 
has but few plants to contend with; so that the acridity serves — 
t 
-to protect it in much the same way as if it had a milky juice. 
