132 THE STEM. 



or one pair ofleaves in germination (118), is complete in its parts, 

 being provided with all the organs of vegetation, namely with root, 

 stem, and foliage. By this time its layer of wood is also manifest 

 (a few vessels being first developed in four or more clusters, 

 around which, principally on the outer side, woody tissue at once 

 begins to appear) ; and the bark a little later exhibits traces of the 

 elements of its three layers. This nascent wood begins to form 

 early in germination. In a large and highly developed embryo it 

 exists before germination. The conversion of young cells of pa- 

 renchyma into vessels and wood-cells either commences in the 

 radicle or stem-part and extends upwards into the cotyledons, when 

 the latter are proportionally little developed ; or, when they are 

 large in proportion (as in the Almond, Fig. 97), it commences in 

 the cotyledons and grows downwards into the radicle. The wood 

 of the rudimentary stem and that of the leaf or leaves it bears are 

 therefore in connection, are parts of the same system. As the 

 root is produced from the lower end of the radicle (Fig. 107), its 

 forming woody tissue extends downwards into it (the primary ves- 

 sels, however, commonly developing as ducts instead of spiral ves- 

 sels), and grow on as that advances by its cellular growth. The 

 leaf or pair of leaves of the second internode by this time begins 

 to appear ; in which, or at the base of which, new vascular and 

 woody tissues originate in the same way, extending through the 

 leaf to form its woody system, or framework, making the woody 

 stratum in the second internode of stem as it lengthens, and 

 then contributing to the increase of the wood of the parent inter- 

 node beneath. This is repeated throughout the whole growth 

 of the season ; each internode forming its own woody system, a 

 portion of which appears separately in the leaf, while that in its 

 stem blends with that of the internodes below to form the gen- 

 eral zone of wood, in the exogenous stem. It is nearly the 

 same in the endogenous stem, except that the wood forms in sep- 

 arate bundles or threads, and these are commingled through the 

 whole circumference of the young stem, instead of the new wood 

 being constantly applied to the outside of that which was first 

 formed. In the endogenous stem, the individual threads or bun- 

 dles which form the wood may often be separately traced from 

 the base of the leaf to their termination, at a considerable distance 

 below. In the exogenous stem, their elements are usually conflu- 

 ent and undistinguishable in the common layer. 



