60 LIFE OF A TREE. 
parts,—a clot or cream, and a thin fluid or whey. 
Singularly enough, this milk-sap has the same 
property; when it is drawn from the plants in 
which it flows, it separates, just as they do, into a 
cream or clot, and a thin fluid or whey. Nor 
can we mix them together again when they are 
once separated in this manner. It is found that 
the milk-sap when fresh drawn, before its sepa- 
ration, consists of a vast number of small globules 
floating in a clear fluid, and thus giving it a milky 
appearance. On standing, the little globules col- 
lect together, and float at the top like cream. It 
is also very remarkable that this fluid is not, like 
common sap, found in all the tissues of the plant, 
but exclusively in a beautiful mesh-work of fine 
tubes, which exist principally in the bark of the 
milk-bearing plants. On this account the disco- 
verer* of them was led to consider that the milk- 
sap was really a sort of vegetable blood, and that 
these tubes were the veins in which it flowed. But 
it does not yet appear that this opinion is correct. 
Although confined to a few vegetable tribes, 
the milk-sap is almost of more value and im- 
portance to mankind than the sap proper. When 
* Prof. Schultz of Berlin. 
