LESSON 24.] 



"WOOD. 



147 



409. In hardwoods, such as Hickory, Oak, and Buttonwood (Fig. 

 345), the walls of these tubes are very thick, as well as dense ; while 

 in soft woods, such as White-Pine and Basswood, they are pretty thin. 



410. "Wood-cells, like other cells (at least when young and living), 

 have no openings ; each has its own cavity, closed and independent. 

 They do not form anything like a set of pipes opening one into an- 

 other, so as to convey an unbroken stream of sap through the plant, 

 in the way people generally suppose. The contents can pass from one 

 cell to another only by getting through the partitions in some way or 

 other. And so short are the individual wood- 

 cells generally, that, to rise a foot in such a tree 

 as the Basswood, the sap has to pass through, 

 about two thousand partitions ! 



411. But although there are no holes (ex- 

 cept by breaking away when old),- there are 

 plenty of thin places, which look like perfora- 

 tions; and through these the sap is readily trans- 

 ferred from one cell to another, in a manner to 

 be explained further on (487). Some of them sm 347 

 are exhibited in Fig. 345, both as looked directly down upon, when 

 they appear as dots or holes, and in profile where the cells are cut 

 through. The latter view shows what they really are, namely, very 

 thin places in the thickness of the wall ; and also that a thin place in 

 one cell exactly corresponds to one in the contiguous wall of the next 

 cell. In the wood of the Pine family, these thin spots are much 

 larger, and are very conspicuous in a thin slice of wood under the 

 microscope (Fig. 346, 347) ; — forming stamps impressed as it were 

 upon each fibre of every tree of this great family, by which it may 

 be known even in the smallest fragment of its wood. 



412. Wood-cells in the bark are generally longer, finer, and 

 tougher than those of the proper wood, and appear more like fibres. 

 For example. Fig. 344 represents a cell of the wood of Basswood, 

 of average length, and Fig. 342 one (and part of another) of the 

 fibrous bark, both drawn to the same scale. As these long cells 

 form the principal part of fibrous bark, or bast, they are named Bast- 

 cells or Bast-Jibres. These give the great toughness to the inner 

 bark of Basswood (i. e. Bast-wood) and of Leatherwood ; and they 



FIG. 346. A bit of Pine-shaving, highly magnified, showing the large circular thin spots 

 of the wall of the wood-cells. 347. A separate wood-cell, more magnified, the varying thick' 

 ness of the wall at these spots showing as rings. 



