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340 HOW CROPS GROW. 
exude from the broken stem of the milk-weed (Aselepias,) 
of lettuce, or of celandine (Chelidonium,) 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 upwards 
through the plant must not be confounded with the normal, 
necessary, and often contrary motion of the nutrient mat- 
ters out of which new growth is organized, but is an in- 
dependent or highly subordinate process by which the 
plant adapts itself to the constant changes that are taking 
place in the soil and atmosphere as regards 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 developes nearly 
alike. In both cases the nutritive matters gathered 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 flow of sap in the plant is confined to the 
vascular bundles, whether these are arranged symmetri- 
cally and compactly, as in exogenous plants, or distributed 
singly through the stem, as in the endogens. This is not 
only seen upon a bleeding stump, but is made evident by 
the oft-observed fact that colored liquids, when absorbed 
into a plant or cutting, visibly follow the course of the 
