OUR TREES IN WINTER. 43 



discussed from the horticultural rather than from a 

 strictly scientific point of view. It is a subject which 

 even the most eminent botanist finds difficult to explain 

 in a simple and satisfactory manner. 



We often hear the woodman settle this question of 

 freezing or not freezing, by asserting that when the trees 

 are cut in winter the chips fly off frozen solid. A writer 

 in a horticultural journal, not long since, sought to prove 

 that the sap in trees became frozen in winter, by stating 

 that the Lime trees on Boston Common froze until the 

 trunk cracked open sufficiently to permit the insertion 

 of his hand to a considerable depth into the trunks of 

 several of these trees. This habit of opening upon the 

 side of the trunk, during extremely cold weather, is com- 

 mon among Lime trees and some other species. Neither 

 of these illustrations, however, proves that the sap 

 freezes. 



An examination of thin slices of any plant, under the 

 microscope, shows it to be composed entirely of cells ; 

 the woody tissue of trees and their bark being formed by 

 the union of cells into tubes which are densely packed 

 together. 



The only portion of the trunk of the tree, which may 

 be considered as living, is that thin layer of cells, less in 

 many cases perhaps than one millimetre in thickness, 

 which lies between the bark and the older wood of the 

 tree. In the botanies this is called the cambium layer. 

 It is chiefly through this layer of cells, that the juices of 

 the tree are to be passed and repassed, which process we 

 call the flow of the sap. Among the cells of this layer 

 are formed the new cells, which are added either to the 

 outer layer of wood, and form the circle of growth for 

 the year, or to the inner surface of the bark, forcing 

 apart its older outer surface, which by the continued 

 pressure becomes broken and rifted as the tree grows. 



