15*2 STEM. 



67. The Heart- wood was, when young Alburnum, and 

 afterwards 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 destruction by the introduction of secretions that 

 become solid matter, which is often insoluble in water, and 

 never permeable to air. 



69. The secretions by which Heart- wood is solidified are 

 prepared in the leave's, whence they are sent downwards 

 through the bark, and from the bark communicated to the 

 central part of the stem. 



70. The channels through which this communication takes 

 place are called Med ullary Rai,s } or Stiver Grain. 



71. Medullary rays are plates of cellular tissue, in a very 

 compressed state, passing from the pith into the bark. 



72. The wood itself is composed of tubes consisting of woody 

 fibre and vascular tissue, imbedded longitudinally in cellular 

 substance. 



73. This cellular substance only developes horizontally ; and 

 it is to it that the peculiar character of different kinds of wood 

 is chiefly due. 



74. For this reason the w r ood of the stock of a grafted plant 

 will never become like that of its scion, although as will be 

 hereafter seen (IV.), the woody matter of the stock must all 

 originate. in the scion. 



75. The stem of an exogenous plant may therefore be com- 

 pared to a piece of linen, of which the weft is composed of 

 cellular tissue, and the warp of fibrous and vascular tissue. 



76. In the Spring and Autumn a viscid substance is secreted 

 between the wood and the liber, called the Cambium. 



77- This Cambium appears to be the matter out of which 

 the cellular horizontal substance of the stem is organised. 



78. In Endogenous stems the portion at the circumference 

 is harder than that in the centre; and there is no separable 

 bark. 



79. Their stems consist of bundles of woody matter, imbed- 

 ded in cellular tissue, and composed of vascular tissue surroun- 

 ded by woody fibre. 



80. The stem is not only the depository of the peculiar 

 secretions of species (67.), but is also the medium through 

 which the sap flows in its passage from the roots into the 

 leaves. 



81. In Exogenous stems (63.) it certainly rises through the 

 alburnum, and descends through the bark. 



82. In Endogenous stems (64) it probably rises through the 

 bundles of wood, and descends through the cellular substance ; 

 but this is uncertain. 



83. Steins have the power of propagating an individual only 



