10 VEGETABLE PHYSIOLOGY. 



occupies the centre of the latter from one extreme to the other. It is 

 a spongy substance called cellular tissue, because it consists of little 

 bao-s or cells. It constitutes the chief of all young shoots, and in them 

 has always a greenish tinge, from these little cells being filled with a 

 fluid of that colour ; but as the shoots advance in age this fluid gra- 

 dually disappears, and ultimately the pith becomes white. It is one 

 of the simplest forms of vegetable tissue, and the most rapid in its 

 development ; it is, therefore, well adapted to the end for which it 

 appears to be chiefly designed, viz., to act as a reservoir of nourish- 

 ment upon which the tender leaves and vessels of immature shoots 

 may feed, till they can obtain a more perfect means of supply. 



Beneath the bark, and only separated from the pith by the medul- 

 lary sheath, (an exceedingly 'delicate membrane, consisting of spiral 

 vessels containing air, and forming a very thin layer round the pith,) 

 we find the layers of wood, each of which is the growth of one year, 

 consequently their number indicates the age of the tree. This only 

 holds good, however, in trees of temperate and extreme northern and 

 southern latitudes, where the powers of vegetation are for a tinae sus- 

 pended; because in the more congenial climate of the tropics the 

 trees experience no such check upon their vital energies, and, conse- 

 quently, their annual growth is less distinctly marked, the wood of 

 one year seeming to pass imperceptibly into that of another. In 

 glancing at the cross section of the stem of the Hazel above cited, it 

 will be "seen that its annual zones are radiated by fine lines passing 

 from the centre to the circumference. These lines are called medul- 

 lary rays. They consist of thin vertical plates, of the same kind of 

 tissue as the pith, of which, in fact, they are a lateral extension, 

 establishing a connexion between the interior of the stem and the 

 bark. The inner and outer sides of each annual layer of wood differ 

 somewhat in their character. That side lying nearest the pith is com- 

 posed chiefly of sap-vessels or ducts, organized in the early part of 

 the summer, from a viscid fluid called cambium, which is secreted, in 

 the spring, between the bark and the wood. Through these vessels 

 the sap, taken up by the roots, ascends lo the leaves. That side of the 

 woody layer lying nearest the bark is of a more compact texture, being 

 composed of woody fibre, elaborated and sent down by the leaves 

 much later in the summer. It is a very firm tissue, and gives to the 

 stem great mechanical strength. 



The bark is the external covering of the stem, and has justly been 

 termed the universal leaf of a vegetable, because in it the proper juice 

 of a plant receives its final elaboration. It is provided with a system 

 of branching vessels analogous to those which are seen on the under 

 surface of a leaf. Through these vessels the elaborated sap is con- 

 veyed, and thence, by means of the medullary rays, to every part of 

 the system. The distribution of the proper juices of a plant furnish a 

 fine display of Divine intelligence. Some of it is expended in the 

 production of new layers of wood, some in the production of new 

 branches ; the leaves require a definite quantity, so also do the 

 flowers and fruits ; a portion is also sent downward to the root, and 



