118 



THE STEM. 



or less flattened by the pressure of the woody wedges, and they 

 serve to keep up the communication between the pitia and the 

 bark. 



210. The First Tear's Growth of an exogenous stem accordingly con- 

 sists of three principal parts, viz. : 1st, a central cellular portion, or 

 Pith ; 2d, a zone of Wood ; and 3d, an exterior cellular portion, or 



Bark. These several 

 parts are displayed 

 in Fig. 189-191, as 

 they occur in a woody 

 stem a year old. 



211. The Pith (ilfe- 

 dulla) consists en- 

 tirely of soft cellular 

 tissue, or parenchy- 

 ma* (51), which is 

 at first gorged with 

 sap. Many stems 

 ' expand so rapidly in 

 diameter during their 

 early growth, that 

 they become hollow, 

 the pith being torn 

 away by the disten- 

 tion, and its remains 

 forming a mere lin- 

 ing to the cavity. 

 In the "Walnut and 

 the Poke (Phytolac- 



* In rare instances a few threads of woody tissue and vessels are found dis- 

 persed tlirough the pith, presenting a somewhat remarkable anomaly. This 

 occurs in Aralia racemosa, and more strikingly in Oxybaphus, Mirabilis or 

 Tour-o'clock, and other Nyctaginaceas. 



FIG. 189- Longitudinal and transverse section of a stem of the Soft Maple {Acer dasycar- 

 pum), at the close of the first year's growth ; of the natural size. 



FIG. 190. Portion of the same, magnified, showing the cellular pith, surrounded hy the 

 wood, and that enclosed by tlie barli 



FIG. 191. More magnified slice of the same, reaxjhing from the bark to the pith : a, part of 

 the pith ; 6, vessels of the medullary sheath ; c, the wood ; dd^ dotted ducts in the wood ; «, 

 annular ducts ; /, the liber, or inner fibrous bark ; g, the cellular envelope, or green bark ; A, 

 the corky envelope ; i, the skin or epidermis ; ^, one of the medullary rays, seen on the trans- 

 verse section. 



