116 



THE STEM. 



portion, or Bark. Fig. 154 represents a section of a woody exoge- 

 nous stem, a year old, of the natural size. Fig. 155 shows a por- 

 tion of the same, magnified, so that the different parts may be dis- 

 tinguished, both on the longitudinal and transverse section : and 

 Fig. 156 is a much more magnified view of a slice of the same, 

 reaching from the bark to the pith. 



193. The Pith (Fig. 155, 156, a) consists entirely of soft cellu- 

 lar tissue, or parenchyma (51), which is at first gorged with the 

 nourishing juices of the plant. These are in time exhausted, leav- 

 ing the older pith dry and light, or mere empty cells, which are of 

 no further use to the plant. Many stems expand so rapidly in di- 

 ameter during their early growth, that they become hollow, the pith 

 being torn away by the distention, its remains forming a mere lining 

 to the cavity, as in Grasses and other herbs ; or else it is separated 

 into horizontal plates, as in the Poke (Phytolacca) and the Wal- 

 nut. Immediately surrounding the pith, and the very earliest part 

 of the longitudinal system to appear, is what is called by the su- 

 perfluous name of 



194. The Medullary Sheath, This consists merely of the earliest 

 formed vessels, already spoken of (190), and which of course stand 

 in a circle immediately surrounding the pith ; but they are seldom 

 if ever so numerous as to form a closed layer, or sheath for the pith. 

 More commonly they appear as a few bundles, one at the inner 

 border of each of the larger and earlier woody wedges. They 

 are mostly of the kind named spiral vessels (60), and it is remark- 



FIG. 151. Plan of a cross-section of a young seedling stem, showing the manner in which 

 the young wood is imbedded in the cellular system. 



FIG. 152. The same at a later period, the woody bundles increased so as nearly to fill the 

 circle. 



FIG. 153. The same at the close of the season, where the wood has formed a complete cir- 

 cle, separating the pith from the bark, except that they are still connected by narrow portions 

 of the cellular system (the medullary rays) which radiate from the pith to the bark. 



