Vegetable Physiology. 145 



nd bark. The pith is a soft, spongy substance, occupying 

 the centre. Under a magnifier, it is found to consist entirely 

 of cellular tissue, containing, when young, a good deal of fluid, 

 but when old becoming dry, and often nearly disappearing. 

 This is the first formed portion of the stem, and is the remain- 

 der of the cellular structure which originally formed the whole, 

 but which has gradually given place to the woody texture, 

 which is deposited on its outside, and has gradually com- 

 pressed it into the centre. The pith of a branch is always 

 an extension of that of the parent branch, and if the latter be 

 cut through, just where a bud is rising from it, the bud will be 

 seen to consist mostly of a prolongation of the central pith. 



Around the pith are disposed the woody layers, at first in the 

 form o( strings, arranged in a circle between it and the bark, 

 and separated from each other by prolongations of pith, which 

 is thus connected with the bark. When the stem grows older, 

 a second circle of woody layers is formed, beneath the bark, 

 and enclosing the first layer, and as it increases in age, circle 

 after circle succeeds in the same manner, till the pithy pro- 

 cesses between becoming narrower in each ring, appear at 

 length merely as lines diverging from the centre, and are then 

 called medullary rays. Their office is to maintain a constant 

 connexion between the interior portion of the stem and the 

 bark. These rays form what is called silver grain, so conspic- 

 uous in several kinds of wood, such as Maple and White 

 Oak, and which add so much to their beauty when polished. 

 In general, in temperate climates, a single ring or layer of 

 woody matter is added each year, so that the age of a tree 

 may be reckoned by counting these in a cross section ; but in 

 tropical climates, where many trees have several successions 

 of leaves yearly, there is reason to believe that a corresponding 

 number of layers is formed. This may account for the great 

 number of layers in the Baobab trees of Senegal, which have 

 been supposed, reckoning their age in this way, to be more 

 than five thousand years old. In most timber trees, the inner 

 and older portion, called the duramen, or heart-wood, is much 

 more hard and dry than the exterior alburnum, or sap-wood ; and 

 sometimes, in the hardest and heaviest woods, as lignum- 

 vitae and black walnut, the line of separation between them is 



