great families follow the papaveraceae, the chicoraceae, the
lobeliaceae, the campanulaceae, the sapoteae, and the cucurbitaceae.
The hydrocyanic acid is peculiar to the group of rosaceo-amygdalaceae.
In the monocotyledonous plants there is no milky juice; but the
perisperm of the palms, which yields such sweet and agreeable milky
emulsions, contains, no doubt, caseum. Of what nature is the milk of
mushrooms?) Since, on examining the distribution of vegetable forms
over the globe, we find that those three families are more numerous in
species in the low regions of the tropics, we must thence conclude,
that a very elevated temperature contributes to the elaboration of the
milky juices, to the formation of caoutchouc, albumen, and caseous
matter. The sap of the palo de vaca furnishes unquestionably the most
striking example of a vegetable milk in which the acrid and
deleterious principle is not united with albumen, caseum, and
caoutchouc: the genera euphorbia and asclepias, however, though
generally known for their caustic properties, already present us with
a few species, the juice of which is sweet and harmless. Such are the
Tabayba dulce of the Canary Islands, which we have already mentioned,*
(* Euphorbia balsamifera. The milky juice of the Cactus mamillaris is
equally sweet.) and the Asclepias lactifera of Ceylon. Burman relates
that, in the latter country, when cow's milk is wanting, the milk of
this asclepias is used; and that the ailments commonly prepared with
animal milk are boiled with its leaves. It may be possible, as
Decandolle has well observed, that the natives employ only the juice
that flows from the young plant, at a period when the acrid principle
is not yet developed. In fact, the first shoots of the apocyneous
plants are eaten in several countries.

I have endeavoured by these comparisons to bring into consideration,
under a more general point of view, the milky juices that circulate in