MOTION OF THE JUICES. 371 



crops (p. 312). The water that thus evaporates from the 

 leaves is supplied by the soil, and, entering the roots, 

 more or less rapidly streams upwards through the stem as 

 long as a waste is to be supplied, but this flow ceases 

 when evaporation from the foliage is suppressed. 



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

 great independent of the vital processes, and compara- 

 tively 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 number of weeks. The escape of liquid from the 

 vine is commonly termed " bleeding," and while this 

 rapid issue of sap is thus strikingly exhibited in compar- 

 atively few cases, bleeding appears to be a universal phe- 

 nomenon, one that may occur, at least, to some degree, 

 under certain conditions with very many plants. 



The conditions under which sap flows are various, 

 according 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 open- 

 ing 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 constant, but fluctuates with the thermometer, 

 being more copious when the weather is warm, and fall- 

 ing off or suffering check altogether as it is colder. 



The stem of the living maple is always charged with 

 water, and never more so than in winter.* This water 



* Experiments made In Tharand, Saxony, under direction of Stoeck- 

 hardt, show that the proportion of water, both in the bark and wood 



