STEM. 



structure of Endogens, the root that of the stem of Exogens 

 nearly ; Ex. Smilax. 



97. The stem of EXOGENS may be distinguished into the 

 Pith, the Medullary Sheath, the Wood, the Bark, and the 

 Medullary Rays. 



98. The PITH consists of cellular tissue, occupying the cen- 

 tre of the stem. 



99. It occasionally contains scattered spiral vessels, which 

 appear to originate in the medullary sheath (104), or scat- 

 tered bundles of vascular and woody tissue, as in Ferula. 



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



101. It serves to nourish the young buds until they have 

 acquired the power of procuring nourishment for themselves. 

 For this purpose it is filled with starch, which, in the process 

 of vegetation, becomes converted into mucilage ; and the latter 

 passes out of the pith into the nascent organs. 



102. It is always solid when first organized ; but in some 

 cases it separates into regular cavities, as in the Walnut, 

 when it is called disciform ; or it tears into irregular spaces, 

 as in Umbelliferous plants. 



103. Its office of nourishing the young parts being ac- 

 complished, it is of no further importance, and dies. 



104. The MEDULLARY SHEATH consists of spiral vessels 63 ". 



105. It immediately surrounds the pith, projections of 

 which pass through it into the medullary rays (132) 63 *. 



106. It is in direct communication with the leaf-buds and 

 the veins of the leaves. 



