MILK-PRODUCING PLANTS. 51 



and hydrocyanic acid.* These mixtures vary not only in 

 the different families, but also in the species which belong 

 to the same genus. Sometimes it is morphine or the nar- 

 cotic principle, that characterises the vegetable milk, as in 

 some papaverous plants ; sometimes it is caoutchouc, as in 

 the hevea and the castilloa ; sometimes albumen and caseum, 

 as in the cow-tree. 



The lactescent plants belong chiefly to the three families 

 of the euphorbiaceae, the urticea?, and the apocineffif. Since, 

 on examining the distribution of vegetable forms over the 

 globe, we find that those three families are more nume- 

 rous in species in the low regions of the tropics, we must 

 thence conclude, that a very elevated temperature contri- 

 butes 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 de- 

 leterious principle is not united with albumen, caseum, 

 and caoutchouc : the genera euphorbia and asclepias, how- 

 ever, though generally known lor 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,^ 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 aliments commonly pre- 

 pared 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 deve- 

 loped. In fact, the first shoots of the apocyneous plants are 

 eaten in several countries. 



* Opium contains morphine, caoutchouc, &c. 



t After these three great families follow the papaveracese, the chico- 

 raceae, the lobeliacese, the campanulaceae, the sapoteae, and the cucurbi- 

 taceoe. The hydrocyanic acid is peculiar to the group of rosaceo-amyg- 

 dalacese. 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 ? 



t Euphorbia balsamifera. The milky juice of the Cactus mannllaris if 

 equally *wert. 



x 2 



