DEVELOPMENT AND FUNCTIONS Of STEM. 75 



There are thus three kinds of stems in the vegetable kingdom, 

 which may be defined generally as follows : — 



1. Exogenous ov Dicotyledonous, having a separable bark ; distinct 

 concentric circles, composed of progressive indefinite vascular bundles, 

 increasing at their periphery, the density diminishing from the centre 

 towards the circumference ; pith enclosed in a longitudinal canal or 

 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 density diminishing from the 

 circumference to the centre ; no distinct pith, no medullary sheath 

 nor medullary rays, the cellular tissue being interposed between the 

 vascular 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 



, medullary sheath nor medullary rays ; conspicuous scars left by the 

 bases of the leaves, stem in some cases entirely cellular. 



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



The stem produces the buds from which branches, leaves, and flowers 

 are developed ; it exposes these organs 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 450 feet. Some Palms 

 attain a height of 200 feet. The trunks of the Baobab and Welling- 

 tonia are sometimes 30 or 40 feet in diameter. 



IChe pith, in its early state (fig. 111^), 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 waUs of the cells 

 which enter into its composition. The medullary sheath, which is the 

 first formed vascular layer (fig. 113 em), 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. 

 This is the part of a Dicotyledonous stem in which these vessels 

 ordinarily occur. The medullary ra^/s (fig. 114 rm) preserve a com- 

 munication 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. 114 c), or the cells be- 

 tween the wood and bark, whose function is to aid m the formation of 



