124 LEAF-BUDS. 



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 

 leaves, whence they are sent downward 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 trom 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 wood of the stock of a grafted plant will never 

 become like that of its scion, although, as will be hereafter seen (IV.), t; 

 woody matter of the stock must all originate in the scion. 



75. The stem of an exogenous plant may therefore be compared 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, imbedded in cel- 

 lular tissue, and composed of vascular tissue surrounded 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. Stems have the power of propagating an individual only by means 

 of their Leaf-buds. If destitute of leaf-buds, they have no power of 

 multiplication, except fortuitously. 



IV. Leaf-buds. 



84. Leaf-buds are rudiments of branches, enclosed within scales, which 

 are imperfectly formed leaves. 



85. All the leaf-buds upon the same branch are constitutionally and ana- 

 tomically the same. 



86. They are of two kinds ; namely, regular or normal, and adventU 

 Onus or latent (119). 



87. Regular leaf-buds are formed at the axillae of leaves 



