370 HOW CROPS GROW. 



siderations demonstrate the essential falsity of the theory 

 itself. 



Flow of Sap in the Plant not Constant or 

 Necessary. We speak of the Flow of Sap as if a rapid 

 current were incessantly streaming through the plant, 

 as the blood circulates in the arteries and veins cf an ani- 

 mal. This is an erroneous conception. 



A maple in early March, without foliage, with its 

 whole stem enveloped in a nearly impervious bark, its 

 buds wrapped up in horny scales, and its roots sur- 

 rounded by cold or frozen soil, cannot be supposed to have 

 its sap in motion. Its juices must be nearly or abso- 

 lutely at rest, and when sap runs copiously from an ori- 

 fice made in the trunk, it is simply because the tissues 

 are charged with water under pressure, which escapes at 

 any outlet that may be opened for it. The sap is at rest 

 until motion is caused by a perforation of the bark and 

 new wood. So, too, when a plant in early leaf is situa- 

 ted in an atmosphere charged witli moisture, as happens 

 on a rainy day, there is little motion of its sap, although, 

 if wounded, motion may be established, and water may 

 stream more or less from all parts of the plant towards 

 the cut. 



Sap does move in the plant when evaporation of water 

 goes on from the surface of the foliage. This always 

 happens whenever the air is not saturated with vapor. 

 When a wet cloth hung out, dries rapidly by giving up 

 its moisture to the air, then the leaves of plants lose 

 their water more or less readily, according to the nature 

 of the foliage. 



Mr. Lawes found that in the moist climate of England 

 common plants (Wheat, Barley, Beans. Peas, and Clover) 

 exhaled, during five months of growth, more than 200 

 times their (dry) weight of water. Hellriegel, in the 

 drier climate of Dahme, Prussia, observed exhalation to 

 average 300 times the dry weight of various common 



