ORGANS OF NUTRITION. 



We have therefore in all exogenous stems but four separate parts, 

 namely, pith, Avood, medullary rays, and bark. We shall now 

 describe these in the order in which they are placed. 

 Fig. 169. 



Fig. 170. 



Fig. IfiO. ITorizontal section from the centre to the circumference of the stem 

 of the Maple, three years old. m. Pitli. t. Spiral ves els. r. Pitted vessels. 

 /. Fibres or wood-cells, c. Cambium layer, s. Epiphlccum ; within which 

 may be observed three cortical layers, marked I p I, p J.p I, corresponding 

 to the three years' growth. The figures 1, 2, 3, refer to the three successive 

 years' growtli of the wood. 



1. Pith or Medulla. (Figs. 169, rn, and 171, a.a.) 

 — This consists essentially of ordinary paren- 

 chyma, and forms a more or less cylindrical 

 or angular column situated at or towards the 

 centre of the stem. Under ordinary circum- 

 stances it is not continued into the root, but 

 it is always in connexion directly with the 

 terminal bud of the stem, and also at first in- 

 directly by the medullary rays with all the 

 lateral leaf-buds ; as the latter, however, con- 

 tinue to develope, their connexion with the 

 central pith is cut off, as Avill be explained 

 hereafter in speaking of their structure and 

 origin (p. 101). The parenchyma of Avhich the 

 pith is composed is generally that kind Avhich is 

 known as regular or dodecahedral {Jigs. 4 and 5), 

 so that when a section is made of it, and ex- 

 amined microscopically, it presents a hexagonal, 

 or polyhedral appearance. When first formed, 

 the pith is commonly of a greenish colour, 

 and the cells of which it is composed are filled ^'o- ^''O- Young 



• .1 n • ^ J. • • i-iii T branch of Walnut 



With fluid containing nutrient substances dis- (jugiaii.«regia) cut 

 solved in it. At this time it appears, there- vertically to show 

 fore, to be in an active condition, and that such * * '*"'' ^* 

 is the case is still fiu-ther proved by the fiict that its cells 

 are often pitted from the deposition of secondary layers in 



