OF THE STRUCTURE OF WOOD AND HERBS. 



81 



Fig. 123. 



Fig. 124. 



In figure 123, is shown, drawn from nature, a trans- 

 verse slice of the cellular tissue of a sugar-cane, so thin as to 

 display only one layer of cells, but a thicker slice of the same 

 plant exhibits a second set of cells behind the first. 



These cells vary greatly in respect to magnitude, and are 

 represented by one naturalist 

 as possessing twenty different 

 sizes, ranging from those as 

 large as a pea to others which 

 are so minute as to require the 



aid of powerful microscopes to perceive them dis- 

 tinctly. 



Hooke examined a thin section of cork, and found 



that no less than sixty cells were placed endwise 



in a line the eighteenth part of an inch in length ; 



more than a million would therefore be comprised 



within the surface of a square inch. 



In most plants the pores of the cellular tissue 



are readily discerned, but in cork they require to 



be highly magnified in order to be clearly seen. 



A thin slice of cork thus magnified is delineated in 



figure 124. The substance is seen to consist of an assemblage of minute cells 



formed of extremely thin membrane. The average size of the cell is about the 



eight hundred and thirtieth part of an inch in diameter. 



PITH. When a cross section of a tree or plant is viewed by the naked eye, 

 it is seen to consist of three parts, the pith, the woody texture, and the bark. 

 The size of the pith varies in different trees, in some being from two to three 

 inches in diameter, and in others from five to six; and of all plants, herbs and 

 shrubs have the greatest quantity of pith in proportion to the other parts. The 

 pith is found to consist entirely of cellular tissue, and the cells are of various 

 sizes. Those of the thistle appear under the microscope as large as the cells of 

 a honey-comb ; those of plum, wormwood, and sumach, are smaller, and the 

 cells in the pith of the apple and pear are still less ; while those of the oak are so 

 minute that one hundred only equal in size a single cell of the pith of the thistle. 

 The size of the cells is not proportioned to that of the pith, for in the plum, the 

 pith of which is less than that of the pear, the cells are from four to five times as 

 large ; and the cells of the pith of the hazel, which is three times smaller than 

 that of the holly, are ten times greater than those in the holly. 



WOOD AND WOODY TEXTURE. The second portion of the plant is the wood 

 or woody texture ; it encircles the pith, and consists, as already stated, of two 

 parts ; bundles of tubes, bound together by cellular tissue. In most trees the 

 vessels are numerous, and when beheld in a cross section are seen to be disposed 



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