LECT. VII.] ANATOMY OF STEMS. 381 



as in almost every old stem, it has acquired the 

 colour of the wood, and is comparatively so small 

 a point in the diameter of a large trunk, it is 

 easily overlooked. In a transverse slice from the 

 centre of a very old Oak, now before me, the 

 medulla is distinctly seen of the same size and 

 figure usually found in the young stem, although 

 its colour is nearly as dark as that of the surround- 

 ing wood, and the cells are altogether obstructed ; 

 and in the stem of a Nectarine, twenty years old, 

 also now before me, the same state of the pith is 

 observable. In both, the cells appear obliterated, 

 even when examined by a good lens ; but when 

 a very thin slice is placed under the microscope, 

 in a drop of pure water, the hexagonal character 

 of the cells is perfectly distinguishable if the 

 section be transverse ; while, if longitudinal, not 

 only the difference of form between the real pith 

 cells and those of the medullary sheath is per- 

 ceptible, but the spiral vessels are seen, filled 

 with a dark-coloured resinous matter. In such 

 stems, therefore, the pith is neither compressed, 

 obliterated, nor converted into wood. But when 

 the ligneous matter is of a loose texture, or, in- 

 stead of forming a continuous circle, it is in 

 separate columns, as in broad-leaved Birth- wort, 

 Aristolochia Sipho, and the divergent rays are very 

 large, the pith, although it is never completely 

 obliterated, yet, is considerably compressed and 



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