JTJ ANATOMY OF STEMS. 



pith to be less essential to the life and growth of a tree, 

 than the bark. Mr. Thompson thinks its intention is to 

 afford the necessary surface for the formation of the first 

 layer of wood ; and likewise that it is of use to give a 

 degree of firmness to the succulent stem and recent 

 shoot, which they would not otherwise possess, before 

 the bark and alburnum acquire sufficient consistence 

 for that purpose. 



The original pith of the young shoot still remains in 

 trees whose wood is of a close texture, as may be seen 

 even in the centre of the oldest Oak, as it is defended 

 by the first cylinder of wood deposited round it, and 

 suffers no material compression by the successive lay- 

 ers. The cells, indeed, appear obliterated even when 

 examined by a good lens, but in a very thin slice 

 placed under the microscope, in a drop of pure water, 

 the hexagonal character of the cells is perfectly distin- 

 guishable if the section be transverse ; while, if longi- 

 tudinal, not only the difference of form between the 

 real pith cells and those of the medullary sheath is 

 perceptible, but the spiral vessels are seen filled with 

 a dark colored resinous matter. In such stems, there- 

 fore, the pith is neither compressed, obliterated, nor 

 converted into wood, as some phytologists have imag- 

 ined. But when the ligneous matter is of a loose tex- 

 ture, or instead of forming a continuous circle, it is in 

 separate columns, as in broad-leaved Birth-wort 

 (Aristolockia Sipho), and the divergent rays are very 

 large, the pith, although it is never completely obliter- 

 ated, yet, is considerably compressed and altered in 

 form, in stems even of a few years' growth. The ul- 

 timate state of the pith, therefore varies ; being regu- 

 lated by the character of the wood which encloses it. 



