FLOWER-GARDENING. 133 



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 pre- 

 pared in the leaves, 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 Medullary Rays, or Silver 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 develops horizontally ; and 

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

 is chiefly due. 



74. For this reason the wood 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 organized. 



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 sur- 

 rounded by woody fibre. 



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

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



