332 HOW CROPS GROW. 



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

 exhaled during 5 months of growth, more than 200 times 

 their (dry) weight of water. The water that thus evap- 

 orates from the leaves is supplied by the soil, and en- 

 tering l^he roots, rapidly streams upwards through the 

 stem as long as a waste is to be supplied, but ceases when 

 evaporation from the foliage is checked. 



The upward motion of sap is therefore to a great de- 

 gree independent of the vital processes, and comparatively 

 unessential to the welfare of the plant. 



Flow of sap from the plant. " Bleeding," It is a 



familiar fact, that from a maple tree " tapped " in spring- 

 time, or from a grape-vine wounded at the same season, a 

 copious flow of sap takes place, which continues for a num- 

 ber of weeks. The escape of liquid from the vine is com- 

 monly termed " bleeding," and while this rapid issue of 

 sap is thus strikingly exhibited in comparatively few 

 cases, bleeding appears to be a universal phenomenon, one 

 that may occur, at least, to some degree, under certain con- 

 ditions with every plant. 



The conditions under which sap flows are various, ac- 

 cording to the character of the plant. Our perennial 

 trees have their annual period of active growth in the 

 warm season, and their vegetative functions are nearly 

 suppressed during cold weather. As spring approaches 

 the tree renews its growth, and the first evidence of change 

 within is furnished by its bleeding when an opening is 

 made through the bark into the young wood. A maple, 

 tapped for making sugar, loses nothing until the spring 

 warmth attains a certain intensity, and then sap begins to 

 flow from the wounds in its trunk. The flow is not con- 

 stant, but fluctuates with the thermometer, being more 

 copious when the weather is warm, and falling off* or suf- 

 fering check altogether as it is colder. 



The stem of the living maple is always* charged with 



