OF BOTANY. XIX 



112. The immediate ftwctions ol" the bark are to protect the young wood from 

 injury, and to serve as a filter through which the descending elaborated juices of a 

 plant may pass horizontally into the stem. 



113. The Medullary Rays or Plates consist of compressed parallelograms of 

 cellular tissue (muriform cellular tissue). 



114. They connect together the tissue of the trunk, maintaining a communica- 

 tion between the centre and the circumference. 



115. They act as braces to the woody fibre and ducts of the wood. 



116. Cambium is the viscid secretion which, in the spring, separates the albur- 

 num from the liber. 



117. It is supposed to be destined to afford a proper pabulum for the descending 

 fibres of the buds. 



118. I believe it exclusively gives birth to the new medullary rays. 



119. As Exogenous plants increase by annual addition of new matter to then 

 outside, and as their protecting integument or bark is capable of distension in any 

 degree, commensurate with the increase of the wood that forms below it, it follows, 

 taking all circumstances into consideration, that there are no assignable limits 

 to the life of an Exogenous tree. 



120. The stem of endogenous plants offers no distinction of Pith, Medullary 

 Rays, Wood, and Bark. 



121. It is formed by the intermixture of bundles of vascular tissue among a mass 

 of cellular tissue, the whole of which is surrounded by a zone of cellular tissue and 

 woody fibre, inseparable from the stem itself, and therefore not bark. 



122. It increases by the successive descent of new bundles of vascular tissue 

 down into the central cellular tissue. 



123. The vascular bundles of the centre gradually force outwards those which 

 were first formed, and in this way the diameter of a stem increases. 



124. The diameter of the stem of an endogenous plant is determined by the 

 power its tissue possesses of distending, and on its hardness. 



125. When the external tissue has once become indurated, the stem can in- 

 crease no further in diameter. 



126. When the tissue is soft and capable of continual distension, there is no 

 more certain limits to the life of an Endogenous than of an Exogenous tree. 



127. Generally, the terminal bud only of Exogenous plants is developed ; but 

 very often a considerable number develope ; Ex. Asparagus. 



128. When a terminal bud only of an Endogenous plant developes, the stem is 

 cylindrical ; Ex, Palms ; when several develope, it becomes conical ; Ex. Bam- 

 boo. 



129. In cellular plants no other stem is formed than what arises from the simple 

 union of the bases of the leaves to the original axis of the bud from which they 

 spring, and which they carry up along with them. This subject is but ill under- 

 stood. 



130. The ascending direction of the stem, upon its first developement, is fre- 

 quently deviated from immediately after. 



131. It often burrows beneath the earth, when it is vulgarly called a creeping 

 root. Sometimes the internodia (137) become much thickened, when what are 

 called tubers are formed ; or the stem lies prostrate upon the earth, emitting roots 

 from its under side, when it is called rhizoma. 



132. If it distend underground, without creeping or rooting, but always retaining 

 a round or oval figure, it is called a cormus. 



133. All these forms of stem are vulgarly called roots. 



134. No root can have either scales, which are the rudiments of leaves, or nodi, 

 which are the rudiments of buds. A scaly root is, therefore, a contradiction in 

 terms. 



135. The ascending axis, or stem, has nodi and internodia. 



136. Nodi are the places where the leaves are expanded and the buds formed. 



137. Internodia are the spaces between the nodi. 



138. Whatever is produced by the evolution of a leaf-bud (142) is a branch. 



139. A spine is the imperfect evolution of a leaf-bud, and is therefore a branch. 



140. All processes of the stem win. 1 , are not the evolutions of leaf-buds, are mere 

 dilatations of the cellular integument of the bark. Such are prickles. {Aculei, 

 Lat.) 



