578 



B O T A N Y. 



along the surface of, and others enter deeply into the 

 ground. 



A ligneous or woody stem supports the branched 

 heads of shrubs and trees, and is composed of three 

 principal parts, viz. the pith, the wood, and the bark. 

 The Pith. This member occupies the centre ol 

 the stem, and constitutes the principal part of the 

 bulk of the seedling, and of every young shoot. It 

 is more or less filled with a spongy matter, easily 

 permeable by fluids. There seems to be no action in 

 the pith, except as a duct, after the first year ; for, as 

 it increases in age, it decreases in volume, and in old 

 trees becomes almost obliterated. This central mem- 

 ber is inclosed in a sheath, to which, if it be a dico- 

 tyledonous tree or shrub, the first concentric layer of 

 wood is attached. In hollow stems, pith is only found 

 at the articulations ; and in jointed stems, which are 

 solid, it is somewhat interrupted at each joint. 



The above remarks refer to the steins of dicoty- 

 ledonous plants ; those of the class monocotyledoneae 

 have no regular pith, this being suffused throughout 

 the whole body of the fibrous stem. 



The Wood, The first layer of this principal mem- 

 ber of a stem is simultaneously produced with the 

 pith which it surrounds. During its growth it appears 

 in three different states ; at first, it is like thin colour- 

 less jellv, in which state it is called cambium; next, 

 it gains a subsistence like gum, showing faint signs of 

 organisation ; and lastly, as perfect wood, called 

 alburnum, having all the fibrous structure, cells, tubes, 

 and consistence of timber. In this manner the dia- 

 meter of all dicotyledonous stems are annually en- 

 largt.d by concentric layers, the pith being in the 

 centre of the whole. These layers of wood are corn-- 

 posed of a mass of ligneous fibres, closely and longi- 

 tudinally arranged, extending from the collet or base 

 to the summit of the trunk, and to that of every 

 branch of the spreading head. The fibres are embed- 

 ded in dense cellular matter, the cells of which are 

 placed horizontally between the bundles, and, being 

 distended in the line of their position, give thickness 

 to the alburnous layer. 



As these layers are perfectly distinct from each 

 other, there being no fibrous INTERJUNCTION, but 

 merely cemented tog-ether by cellular matter and 

 concreted sap, it is necessary there should be some 

 common tie by which the whole column of concentric 

 layers should be held together. This is accomplished 

 by the convergent partitions of dense cellular matter, 

 which reach from the bark to the pith in right lines 

 through each layer. These perpendicular partitions 

 are called "silver grain," and are those wavy lines 

 so visible on oak timber when cut into pannels. 



The number of the layers, reckoning from the pith 

 to the bark, on one side of a transverse section of the 

 butt or trunk, indicates the age of the tree exactly ; for 

 the layers never run into each other, nor do they 

 increase or diminish after they are once imposed. 



After the tree has passed its mature age, it at last 

 begins to decay ; the first imposed layers next the 

 pith fail first ; and this decay at the heart extends 

 outwardly, till the trunk becomes a hollow cylinder, 

 when the whole is laid prostrate by the wind. This 

 happens sooner or later, according to the durability of 

 the timber. Some kinds, from the light porous cha- 

 racter of the wood, and aqueous quality of the sap, 

 perish in a few years ; others, from the density of 

 the grain, and preservative quality of its concreted 

 juices, resist decay for many years. 



The Bark. The seedling rises from the ground 

 with its coat of bark, consisting of a layer of green 

 parenchymous matter, covered by a thin cuticle. 

 This ever remains on the exterior of the greater 

 number of trees, and is distended as the internal 

 growth increases. Some few trees and shrubs, as the 

 plane, arbutus andrachne, and others, discharge their 

 bark every third or fourth year, but on the greater 

 number the outer bark remains, and is either rent 

 into longitudinal irregular furrows, or stretched 

 horizontally. At the end of the second year, the 

 second layer of bark within the first becomes visible, 

 and takes the name of lihcr, a new layer of liber being 

 formed within the former in ever}' year during the 

 life of the tree. The diameter of the tree is thus 

 increased by a new layer of alburnum, or white wood, 

 and a new layer of liber ; of course they are always 

 equal in number to each other. The liber, however, 

 of many trees, is so thin, and of so open a fabric, that 

 the layers cannot, without maceration, be easily se- 

 parated from each other ; but others, as those of the 

 lime-tree, are so entire and tenacious, that they are 

 easily separable, and woven into Russia mats. 



The new layers of wood and bark are the principal 

 channels for the motions of the sap, it being always 

 more copious in those members than in any other. 

 Whatever the qualities of the sap of any tree may be, 

 we generally find them more powerful and in greater 

 quantity in the bark than in the wood ; hence the 

 many purposes to which bark is employed iu me- 

 dicine and in the arts. 



The stems or trunks of monocotyledonous plants 

 are very differently formed. Instead of additional 

 layers of new matter being deposited on the outside 

 E.vogcncE\ the new accretion is formed, and proceeds 

 from within (Endogena:}, This manner of growth is 

 exemplified in the paltns and other plants belonging 

 to the same class, but the most familiar instance is 

 ;hat of the common asparagus. In this plant it will 

 je observed (if we consider one division of it) that it 

 las a system of thick fleshy roots, fringed with 

 ibres, proceeding from the collet, or crown. From 

 the centre of the latter the stem takes its rise, and is 

 almost as large on its first appearance above ground 

 as it is ever afterwards. Its development is evidently 

 an elongation of its previously existing organisation, 

 and surely not consolidated by any fibrous processes, 

 which, during growth, descend from the branched 

 :iead. 



The stems of all the other orders of monocoty- 

 edoneae are similar in their development. Whether 

 we examine the peduncles of bulbs, the stems of the 

 ridcae and musaceee, or the culms of the graminea?, 

 we find they all gain elevation by an elongating 

 >rocess, and by the simple distension and subdivision 

 of the cellular and vascular apparatus, without addi- 

 tion of any new matter save the incremental elements 

 which they extract from the earth and atmosphere. 

 Here it may be necessary to premise, that in con- 

 sidering or investigating vegetable structure in gene* 

 ral, we should always distinguish the mere distension 

 of the cellular and vascular fabric of plants from their 

 elements. The former are rudimental, the latter ac- 

 cessory ; the first pre-exist before enlargement, the 

 ast are imbibed during, and to assist enlargement. 



The development of other monocotyledonous plants 

 may be noticed here. There are two very remarkable 

 constitutional differences ; in some the fructification 

 s terminal, that is, placed on the point of a central 



