COMPOSITION OF THE JUICES. 339 
1 2 3 4 5 
Volatile substance - 1.45 0.60 0.30 0.25 0.21 
Ash - - - - - - 1.58 1.56 1.18 0.70 0.60 
Total - - - - 8.03 2.16 148 0.95 0.81 
The water which streams from a wound dissolves and 
carries forward with it matters, that in the uninjured plant 
would probably suffer a much less rapid and extensive 
translocation. From the stump of a potato-stalk would 
issue by the mere mechanical effect of the flow of water 
substances generated in the leaves whose proper movement 
in the uninjured plant- would be downwards into the 
tubers. 
Different kinds of sap.—It is necessary at this point 
in our discussion to give prominence to the fact that there 
are different kinds of sap in the plant. As we have seen, 
(p. 267,) the cross section of the plant presents two kinds 
of tissue, the cellular and vascular. These carry different 
juices, as is shown by their chemical reactions. In the 
cell-tissues exist chiefly the non-nitrogenous principles, 
sugar, starch, oil, etc. The liquid in these cells, as Sachs 
has shown, commonly contains also organic acids and acid- 
salts, and hence gives a blue color to red litmus. In the 
vascular tissue albuminoids preponderate, and the sap of 
the ducts commonly has an alkaline reaction towards test 
papers. These different kinds of sap are not, however, 
always strictly confined to either tissue. In the root-tips 
and buds of many plants (maize, squash, onion) the young 
(new-formed) cell-tissue is alkaline from the preponderance 
of albuminoids, while the spring sap flowing from the 
ducts and wood of the maple is faintly acid. 
In many plants is found a system of channels (milk- 
ducts) independent of the vascular bundles, which contain 
an opaque, white, or yellow juice. This liquid is seen to 
