THE MILK-SAP OF PLANTS. 59 
ing Pampas, when Cayman and Boa sink into a 
death-like sleep in the dried up mud, the wild ass 
alone, traversing the steppe, knows how to guard 
against thirst: cautiously stripping off the danger- 
ous spines of the Melocactus with his hoof, and 
then in safety sucking the cooling vegetable juice.” 
Some races of plants, however, possess a large 
amount of vegetable juice which widely differs 
from sap, properly so called, in both its ‘appear- 
ance and properties. In the common lettuce and 
in the dandelion, we have good specimens of this 
peculiar fluid or milk. Every one knows what hap- 
pens when we break off the head of a dandelion- 
flower,—how an abundance of milk-white juice 
comes pouring up from the stalk, which possesses 
a very peculiar smell, and leaves a sticky feeling 
on the fingers. We shall call this the milk-sap of 
plants. It is not, like ordinary sap, common to all 
plants, but is peculiar to two or three families, 
When we come to examine into the nature of 
this fluid, we find in it a most startling resem- 
blance to the blood of animals, or perhaps more 
closely to their milk. It is well known that 
either blood or milk, when drawn from the body 
and allowed to stand, presently separates into two 
