MOTIOX OF THE JUICES. 379 



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, p. 304), independent of the vascular bundles, 

 which contain an opaque, white, or yellow juice. This 

 liquid is seen to exude from the broken stem of the milk- 

 weed {Asclepias), of lettuce, or of celandine (Chelidon- 

 ium), and may be noticed to gather in drops upon a 

 fresh-cut slice of the sweet potato. The milky juice 

 often differs, not more strikingly in appearance than it 

 does in taste, from the transparent sap of the cell-tissue 

 and vascular bundles. The former is commonly acrid 

 and bitter, while the latter is sweet or simply insipid to 

 the tongue. 



Motion of the Nutrient Matters of the Plant. — 

 The occasional rapid passage of a current of water up- 

 wards through the plant must not be confounded with 

 the normal, necessary, and often contrary motion of the 

 nutrient matters out of which new growth is organized, 

 but is an independent or highly subordinate pi:ocess by 

 which' the plant adapts itself to the constant changes 

 that are taking place in the soil and atmosphere as re- 

 gards their content of moisture. 



A plant supplied with enough moisture to keep its tis- 

 sues turgid is in a normal state, no matter whether the 

 water within it is nearly free from upward flow or ascends 

 rapidly to compensate the waste by evaporation. In- 

 both cases the motion of the matters dissolved in the sap 

 is nearly the same. In both cases the plant develops 

 nearly alike. In both cases the nutritive matters gatli-' 

 ered at the root-tips ascend, and those gathered by the 

 leaves descend, being distributed to every growing cell ; 

 and these motions are comparatively independent of, and 

 but little influenced by, the motion of the water in which 

 they are dissolved. 



The upward ^ow of sap in the plant is confined to the 



