58 DEVELOPMENT AND FUNCTIONS OP STEM. 



medullary sheath, with cellular prolongations in the form of medullary 

 rays. 



2. Endogenous or Monocotyledonous, having no separable bark; no 

 distinct concentric circles ; vascular bundles progressive and definite, 

 not increasing at their periphery, the solidity diminishing from the 

 circumference to the centre; no distinct pith, no medullary sheath nor 

 medullary rays, the cellular tissue being interposed between the vas- 

 cular bundles. 



3. Acrogenous or Acotyledonous, having no separable bark; no con- 

 centric circles; vascular bundles simultaneous, forming an irregular 

 circle; additions being made to the summit; no distinct pith, no medul- 

 lary sheath nor medullary rays; conspicuous scars left by the bases of 

 the leaves. 



Formation of the different parts of Stems, and their special Functions. 



108. The stem bears the leaves and flowers, exposes them to the 

 atmosphere and light, conveys fluids and air, and receives secretions. 

 Stems vary much in their size, both as regards height and diameter. 

 Some oaks in Britain have a height of nearly 120 feet ; forest trees in 

 France have attained to 120 and 130 feet, and in America even to 

 150 feet; while palms are frequently still higher. The trunks of 

 some Baobabs in Senegal (Adansonia digitatd), are said to be 30 feet 

 in diameter. 



109. The pith, in its early state (fig. 94 p), is of a greenish colour, 

 and contains much fluid, which is employed in the nourishment of 

 the young plant. After serving a temporary nutritive purpose, it 

 becomes dry, or disappears by rupture and absorption of the walls of 

 the cells which enter into its composition. The medullary sheath (fig. 

 96 e TO), keeps up a connection between the central parts of the stem 

 and the leaves, by means of spiral vessels, which seem to be concerned 

 partly in the conveyance of air. The medullary rays (fig. 97 r m), 

 preserve a communication between the bark and the pith. The cells 

 of which they are composed, are concerned in the production of leaf 

 buds, and they assist in the elaboration and conveyance of secretions. 

 They have a direct connection with the cambium cells (fig. 97 c), or 

 the cells between the wood and bark, whose function is to aid in the 

 formation of new wood. The bark (fig. 97 fc, e c, p), protects the 

 tender wood, conveys the elaborated sap downwards from the leaves, 

 and is the part in which many valuable products, such as gum, tan- 

 nin, and bitter principles, are formed and deposited. The vascular 

 bundles (fig. 97 f I, v p), convey the sap from the root to the leaves. 

 This function is carried on during the life of the plant by the annular 

 vessels and the porous vessels, as well as other kinds of spurious fibro- 

 vascular tissue ; but in the woody fibres it ceases at a certain epoch, 



