218 THE SECOND BOOK OF BOTANY. 



year's growth (Fig. 391). Passing outward from the 

 medulla, or pith, marked M, we come first upon the 

 spiral ducts of the woody bundles, marked T. They 

 form a sort of sheath around the 

 pith, only broken by the medul- 

 lary rays (KM), and, as they en- 

 close it, they are called the me- 

 dullary sheath. This portion 

 of the woody bundle is contin- 

 uous with the petiole and frame- 

 work of leaves. 



EXPERIMENT. Divide the 

 bark and most of the wood of 



a young shoot by a circular cut, and gently pull it 

 asunder ; you may detect this sheath by the stretched 

 and broken spiral threads of its fibres. 



Outside of the medullary sheath, observe the lig- 

 neous fibres, or zone, of true wood. Encircling this 

 is the cambium layer. Up to this point, excepting 

 its somewhat less density, the herbaceous and the 

 woody stems are alike. It is in the portion external 

 to the cambium that we come upon differences. The 

 cambium and bark, it will be seen, are important in 

 plants that are to live over to another year, while 

 they are of little account to the herb, which dies in 

 autumn. In herbaceous stems, as the melon, there- 

 fore, the bark consists of simple parenchyma, like 

 that of pith, except that it is of a green color ; but 

 in woody stems, as the maple, it takes on a much 

 higher development, and presents important differ- 

 ences of structure. All the while that wood is 

 forming, bark is also being made. That portion of 

 it next the cambium is wrought into woody tissue, 



