132 FLOWER-GARDENING. 



III. STEM. 



57. The Stem is that part of a plant which is developed 

 above ground, and which took an upward direction at the 

 period of germination. 



58. In consists of a woody axis, covered by bark having 

 etomata (131) on its surface, bearing leaves with leaf-buds in 

 their axilla3, and producing flowers and fruit. 



59. The points where leaves are borne are called Nodi ; the 

 spaces between the leaves are Internodia. 



60. The more erect a stem grows, the more vigorous it is ; 

 and the more it deviates from this direction to a horizontal or 

 pendulous position, the less is it vigorous. 



61. Some stems are developed under ground, such as the 

 Tubers of the Potato and the Cormus of the Crocus ; but they 

 are known from roots by the presence of leaves, and regular 

 leaf-buds, upon their surface. 



62. Stems increase in diameter in two ways. 



63. Either by the addition of new matter to the outside of 

 the wood and the inside of the bark ; when they are Exoge- 

 nous ; ex. Oak. 



64. Or by the addition of new matter to their inside ; when 

 they are Endogenous ; ex. Cane. 



65. In Exogenous stems, the central portion, which is harder 

 and darker than that at the circumference, is called Heart 

 Wood ; while the exterior, which is softer and lighter, is called 

 Alburnum, or Sap Wood. 



66. The inside of the bark of such stems has also the tech- 

 nical name of Liber. 



67. The heart wood was, when young, Alburnum, and after- 

 wards changed its nature by becoming the receptacle of certain 

 secretions peculiar to the species. 



68. Hence the greater durability of heart wood than of sap 

 wood. While the latter is newly formed empty tissue, almost 

 as perishable as bark itself, the former is protected against 



