VEGETABLE PHYSIOLOGY. 19 



pruned a tree, or shortened a growing twig, must have observed 

 that the part above the last leaf always shrivels and dies, while 

 all below it continues to live and increase in diameter. 



6. The lignum, or heart wood, which, when present, is seated 

 below the alburnum, seems principally intended, (like the bones 

 of animals,) to give solidity, form and support to the more solid 

 parts of the plant, and is mostly observable in old trees. 



Substantial as is the wood or ligneous part of a tree, it is 

 nevertheless so far from being an essential part, that many plants 

 are without it. The arundaceous plants, as the reeds and the 

 grasses, and, indeed, all the gramina, are naturally hollow. 



7. The medulla, or pith, which is a soft and spongy, but 

 often succulent substance, occupying the centre of the root, stem 

 and branches, and extending in the direction of their longitudinal 

 axes, in which it is inclosed as in a tube. In its structure it is 

 exactly similar to that of the cellular tissue of thebark ; being 

 composed of an assemblage of cells, containing a watery and 

 colorless fluid. Its form is regulated by that of the cavity it fills, 

 which in the majority of instances is nearly circular ; but to this 

 there are many exceptions. Thus in the horizontal section of 

 a young stem of the elder (Sambucus) and the plane (Platanus), 

 we find it circular, but furrowed by the bundles of the spiral 

 vessels of the part that surrounds it. In the ivy and ash it is 

 oval ; irregularly oval and furrowed in the plane ; triangular in 

 the oleander (JVerium Oleander) ; pentangular in the European 

 oaV;;. ( Querpis Robur) ; four-sided, with the angles obtuse, or 

 tetragonal, in the common lilac, and yellow-flowering horse 

 chesnut (JEsculus Flava) ; and pentagonal in the walnut 

 (Juglans regia). But besides the diversities of form which the 

 pith presents, it varies in diameter in other respects. In the 

 young tree, of a few inches in height, it is smallest at the basis of 

 the stem, largest in the middle, and smaller again at the summit ; 

 and in the growth of each future year, nearly the same variations 

 in its diameter are observable. In trees more advanced, pressed 

 and acted upon by the heart wood, it begins to diminish ; and in 

 very old trees it disappears altogether. 



Linnaeus attributes great importance to the pith, and erroneously 



