TISSUES OF PLANTS. 165 



bast, and that of the stem, in which the cells are similar but 

 shorter, is called woody fibre. The tubes, which are formed 

 as already described, are styled ducts or vessels, and, as they 

 are mingled with woody fibre in the annual growth of timber, 

 the ordinary combination of these is named fibro-vascular tis- 

 sue. When these elongated cells and ducts are straight and 

 parallel, as in the chestnut, the wood of which they are the 

 warp splits easily, but when they are interlaced and blended 

 irregularly, the longitudinal grain of the wood will resemble 

 that of the elm. 



Every seed and every young plant consists wholly of cel- 

 lular tissue, but with the development of leaves is combined 

 the growth of fibro-vascular tissue. The first vessels to ap- 

 pear in the plantlet are arranged in a circle around a column 

 of jtissue, which remains loose and soft, and after the first 

 season dries up and dies. This is called the pith and seems 

 essential to the life of every woody stem and branch during 

 its infancy, although its special function is unknown. Between 

 the vessels around the pith may be seen the rays of cellular 

 tissue which ultimately become hard and firm, and which 

 unite in bonds, never broken except Ly some external force, 

 the inside of the stem with the outside of the bark. These 

 rays make up the woof and have much to do with the distinc- 

 tive peculiarities of different sorts of timber. 



Immediately outside the vessels enclosing the pith grows a 

 layer of woody fibre, upon which, in a more or less developed 

 state, according to the season, is a layer of organizable mate- 

 rial, called cambium, which may be regarded as the seat of 

 life in the plant. 



Next to the cambium, and united to the wood by the rays 

 from the pith, is the bark, consisting of three layers. The 

 inner or fibrous layer is formed by bast cells and firm cellular 

 tissue. In plants with a milky juice this layer is permeated 

 by irregular reticulated canals called the laticiferous or milk- 

 bearino^ vessels. What service these vessels or their contents 

 render to the growing plant is unknoAvn. The milky fluid, 

 however, often furnishes to man some valuable product. 

 Thus the milk of the several species of cow-tree is a nutri- 

 tious and agreeable beverage, that of the poppy-head yields 

 opium, and the juice of several tropical trees supplies the 



