STEMS 19 



heart-wood or duramen. A thinner zone of lighter colored 

 softer wood called sap-wood or alhurnuni surrounds the heart- 

 wood. The pith in a mature plant serves no useful purpose. 

 The heart-wood is but a support for the remainder of the tree, 

 as its vessels no longer conduct water and its cells contain no 

 living matter. The sap-wood conducts the sap from roots to 

 leaves and many of its cells are still living; starch, sugar, and 

 other compounds are often stored in the sap-wood. 



30. Cambium. — Next outside the sap-wood in the stem is a 

 layer of soft formative tissue from which new wood and new 

 bark originate, the cambium layer. The cells in this layer are 

 filled with nutritive sap and in the period of greatest growth 

 activity have so slight a hold that the bark easily peels. The 

 cambium contains most of the growing cells in stems. Besides 

 being rich in nutritive substances, these cells contain the proto- 

 plasm necessary to cell division and consequent growth. The 

 cambium layer makes possible, or at least easy, the horticultural 

 method of propagation in which a part of one plant is severed 

 and made to unite with a part of another; this operation is 

 grafting. The part removed to be inserted in another stem is 

 the cion or graft. The plant in which the cion is set is the 

 stock. It is necessary in grafting that the cells of the cam- 

 bium layers of stock and cion come in close contact, after which 

 the two parts become as one through the union of cells. 



31. The bark. — The coverings of a woody stem external to 

 the cambium are roughly designated as hark. In the plants now 

 under discussion the bark may be divided into four easily dis- 

 tinguishable structures. The part of the bark next to the cam- 

 bium which consists of long strong fibers is the inner hark or 

 hast. Next outside is a layer of tissue the cells of which con- 

 tain green coloring matter or chlorophyll; this layer is the 

 middle hark or green hark. The outer layer of bark is made 

 up of corky cells which contain no chlorophjdl, but by reason of 

 other colored contents give color, each according to species, to 

 the stems and twigs of trees and shrubs ; this is the corky layer 

 or outer hark. The fourth and outermost covering, made up of 

 empty cells, is the skin or epidermis. The outer bark on many 

 fruit-trees is rent and torn by growth from within, worn by 

 the weather, and is thrown off in fragments so that only traces 



