340 HOW CEOPS GEOW. 



exude from the broken stem of the milk-weed {Asclepias,) 

 of lettuce, or of celandine {ChelidoniuTn,) 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 tlie 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 sujjplied 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 



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