40 PBACTIGAL BOTANY. 



The Medullary Sheath, surrounding the pith, is a deli- 

 cate tissue, consisting of spiral vessels. 



The Wood consists of woody fibres and ducts, arranged 

 in concentric circles. 



The Bark is the outermost of the concentric layers, 

 consists chiefly of parenchyma, and is lined by peculiar 

 wood-cells, which we call host-cells. Hence these bast- 

 cells form the inner harh {liber, or endophloeum). The 

 outer layer of the bark, consisting of parenchyma, is di- 

 vided into a green layer {green harh, or mesophloevm) and 

 a corky layer {brown hark, or epvphZoeum), the latter being 

 the outer stratum. The bark, as a whole, is invested by 

 the epidermis. 



Pith and Bark are brought in communication by the 

 medullary rays — narrow plates of parenchyma (and 

 as such they present themselves when seen on .verti- 

 cal sections of the stem). In cross-sections they appear 

 merely as narrow lines. They mn from the pith to "the 

 bark on all sides and make the sil/ver-grain of the wood. 

 (PI. II., 29.) 



83. Cross-sections of the stems of Endogens exhibit 

 no central pith, no distinct, separable bark (whicE, 

 however, is present), and no layer or ring of wood be- 

 tween both, the latter being scattered thronghout the 

 whole in the form of threads or bundles. (PI. II., 28.) 

 In Endogens the bark is constantly found in the cir- 

 cumference of the woody bundles. The section of each 

 thread or bundle exhibits wood within (in or near the 

 axis), elongated cells without (in the circumference), and 

 a parenchyma, which is intermixed with elongated and 

 punctuated cells between. The lengthened cells consti- 

 tuting the outermost layer, are bast-cells, and the punctu- 

 ated cells in the middle stratum a sort of liber. On the 



