COMPOSITION OF THE JUICBS. 339 



12 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 - - - - 3.03 2.16 1.48 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 suifer 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 iininjured 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 j)reponderance 

 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 iuice. This liquid is seen to 

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