OF THE DIFFERENT TRIBES OF PLANTS. 21 



squarish or subhexagonal apertures, and the other cellules being seen on the 

 edge of their plane, present the appearance of opaque lines. The latter are 

 the medullary rays, which proceed from the medulla or pith, and run out in 

 the direction of the bark. There may be more cylinders of woody tissue 

 inclosing each other, the last being inclosed by the bark, of which it is un- 

 necessary to describe the structure. 



In the transverse section, the medullary rays of the ring next the pith, 

 are usually seen to run out to the bark, through all the successive rings, 

 but the second ring is observed to contain a number of medullary rays ori- 

 ginating at its inner margin, and running out through the others. The 

 same is the case with all the successive layers. The only vessels seen in 

 many species are placed on the outer margin of the pith ; but in others 

 there are also large vessels or lacunae scattered in the woody layers. 



We must now examine the appearances presented by the elongated cel- 

 lules of the woody tissue in the longitudinal sections of the stem. 



Fig. 1. Exhibits a transverse section of part of a woody layer of a trunk 

 of Pinus syhestris. Part of the outer margin of the preceding layer is 

 also represented. Several medullary rays are seen. 



Fig. 2. Represents a very thin slice, cut parallel to the medullary 

 rays, in the direction of the length of the stem. The form of the elonga- 

 ted cellules is exposed, and between them are seen the intercellular canals. 

 The lines running across the elongated cellules are portions of the me- 

 dullary rays. On the walls of the cellules are disposed, in a single series, 

 at a greater or less distance from each other, circular substances or vacuities, 

 which some suppose to be glands, and others to be pores. They usually 

 present the appearance of a double circle. 



Fig. 3. If we obtain a very thin slice, by cutting parallel to the bark, 

 in the direction of the length of the stem, we observe the same general ar- 

 rangement as in Fig. 2. Here, however, the medullary rays have been cut 

 across, and present a different appearance. Four of them are seen in the 

 figure. The walls of the elongated cellular tissue are also found to be en- 

 tirely destitute of pores. 



Figs. 4, 5, 6. Represent similar sections of the Spruce Fir. 



