XY111 FIRST PRINCIPLES 



when it is called Endogenous. In cellular plants it is formed by the union of the 

 bases of the leaves, or by simple elongation or dilatation where no leaves or buds 

 exist. 



81. The stem of Exogenous plants may be distinguished into the Pith, the Me- 

 dullary Sheath, the Wood, the Bark, the Medullary Rays, and the Cambium. 



82. The Pith consists of cellular tissue, the vesicles of which are in a slightly 

 compressed state ; it occupies the centre of the stem. 



83. It never alters in diameter after it is once formed. 



84. It is produced by the elongation of the axis upwards. 



85. It serves to nourish the young buds until they have acquired the power of pro- 

 curing nourishment for themselves. 



86. The Medullary sheath consists of spiral vessels and ducts. 



87. It immediately surrounds the pith, projections of which pass through it into 

 the medullary rays (113). 



88. It is in direct communication with the leaf-buds and the veins of the leaves. 



89. It carries upwards the fluid absorbed either immediately from the earth, or 

 through the intervention of the alburnum (101), and conducts it into the leaves. 



90. The Wood lies upon the medulllary sheath, and consists of concentric 

 layers. 



91. It is formed by the successive adhesion of the descending axes of the buds, and 

 by the distention or increase of the cellular tissue of the medullary rays. 



92. The first concentric layer lies immediately upon the medullary sheath and 

 pith, and consists of woody fibre and ducts. 



93. Each succeeding concentric layer consists of an interior stratum of cellular 

 tissue, and an outer stratum of woody fibre and ducts. 



94. Therefore, all the concentric layers that succeed the first may be considered 

 to consist of wood and pith, and to be the same as the first, with the exception of 

 the absence of a medullary sheath. 



95. A concentric layer, once formed, never alters in dimensions. 



96. Each concentric layer, which is distinctly limited, is the produce of one 

 year's growth. 



97. Therefore, the age of an Exogenous plant may generally be known by the 

 number of concentric circles of the wood. 



98. The secretions of plants are deposited first in the oldest concentric layers ; 

 while those layers which are most recently formed are either empty, or contain but 

 a slight deposit. 



99. When the tissue of the concentric layers is filled with secretions, it ceases to 

 perform any vital functions. 



100. The dead and fully formed central layers are called the heart-wood. 



101. The living and incompletely formed external layers are called the alburnum. 



102. Upon the outside of the wood lies the Bark, which, like the wood, consists 

 of concentric layers. 



103. Each concentric layer is composed of woody fibre and ducts, covered exter- 

 nally by a layer of cellular tissue. 



104. The woody fibre and ducts constitute the liber. 



105. The exterior cellular tissue constitutes the cellular integument or ejiiih rmis . 



106. The concentric layers of the wood and bark arc the reverse of each other, 

 the former increasing externally, the latter internally ; the former having a zone of 

 cellular tissue inside, and of woody fibre and ducts outside; the latter having a 

 zone of woody fibre with a few ducts inside, and of cellular tissue outside. 



107. The concentric layers of the bark are formed at the same period, and under 

 the same circumstances, as those of the wood. 



108. Therefore, the number of concentric layers in the one or the other is the 

 same. 



109. But while the concentric layers of the wood are imperishable except from 

 disease, those of tin; bark arc continually destroyed by the distension of the stem : 

 and hence the bark is always perishing naturally, while the wood sustains no 

 loss. 



110. The secretions of a plant arc often deposited in the bark in preference to 

 any other part. 



111. Hence chemical or medicinal principles are often to be sought in the hark 

 rather than in the wood. 



