prineipal bulk of the trunk and branches. It consists of conoenlric layers 

 or rather cylinders, composed chiefly of vascular tissse. In young- shoots, 

 one year old, there is only one layer, which lies upon the medullary sheath. 

 In each succeeding year there is another layer formed. Hence the age of a 

 stem of this kind may be known by counting the number of concentric vascular 

 cylinders in the wood. 



" The external woody layers next the bark are called the alburnum, and 

 differ from the internal layers or true wood in being younger, softer, more 

 succulent, and of a lighter colour. 



•' The true wood or heart wood is formed by the inner layers of the albur- 

 num, which gradually acquire a greater degree of hardness : the transition 

 from alburnum to true wood is, however, almost imperceptible. 



" A new layer of alburnum is formed annually next the bark : it is pushed 

 inwards, and becomes more compact by the pressure of each succeeding 

 annual layer, till at last it becomes almost solid, the sides of the vessels and 

 cells being squeezed together : hence the greater hardness of such trees in 

 the centre. Thus the wood in such stems is eiogenout, or growing at its outer 

 surface. 



" Medullary Rats. The fasciculi, or bundles of vessels which compose 

 the cylindrical layers, are separated at different points by masses of cellular 

 tissue, extending from the centre or pith towards the circumference, causing 

 an appearance of alternate rays of vessels and cellular tissue. These are 

 called medullary rays (from their radiated appearance), medullary pro- 

 longations or insertions, or, the tiiver "rain. The medullary rays most pro- 

 bably convey the proper juice (the descending 6ap) from the bark to the 

 interior of the stem. 



"The wood gives passage to the sap from the root to the buds and leaves, 

 and contains many of the secretions of the plant. The sap rises chiefly 

 through the alburnum (the vessels of which, being young, soft, and not com- 

 pressed, are well adapted for the passage of fluids ), and is conveyed to the 

 leaves, there to undergo changes which render it fit to promote the growth of 

 the plant. Little is known with respect to the particular function of each 

 part of the wood, excepting the alburnum, the latest formed layer of which 

 gives passage upwards to the sap. Thus it would seem that the new layer of 

 alburnum is formed for the nourishment of the plant, by conveying the sap to 

 the leaves ; and this explains why a serious injury of this part is so fatal. 

 When it is destroyed, its office is very imperfectly performed by an old layer 

 filled with secretions, or hardened, and with the vessels contracted by pres- 

 sure. But a tree may live and flourish with a large piece of the bark and 

 alburnum decayed and removed, if not wanting all round. There is an 

 instance of this in a venerable Hawthorn tree near Edinburgh, in which there 

 is a large and deep scar on one side, patched up with stone and lime. 



"The Pith or Medulla. The pith is in the centre of the stem, and is 

 contained in the medullary iheath or canal, which is composed of vessels, chiefly 

 spiral, disposed in a longitudinal direction. In all plants the pith consists of 

 cellular tissue alone, and is of a light and spongy character. The cells are, 

 in general, very regular, and hexagonal in section: in the young shoots of 

 frees, and in herbaceous plants, these cells are filled with aqueous juices, 

 which disappear as the plant grows older, and then they contain gas alone. 



" The pith, it is supposed, nourishes the young wood and the buds during 

 the first year of their existence ; and it has been observed that it retains its 

 moisture for a longer period near the terminal bud, and at the parts where 

 branches are given off. 



" Such is the structure of the stems of that very large class of plants which 

 constitutes the third division. They are found only in Dicotyledonous plants 

 (plants with two lobes in the seed ) as the pea or lupin, and their leaves 

 present in the veins an irregular reticulated appearance. They are called 

 Faogeuouf, because the wood, which is the principal part of them, increases in 

 diameter by the addition of new matter at its external surface. All fhe trees 

 of this country arc Exogenous. 



To be continued. 



