lovejoy: anatomy of g^rtneria. 283 



dition all the walls are deeply pitted with plain pits, and have 

 much the appearance of stone cells. (Figs. 4 and 7.) In the 

 youngest condition of these pith cells, before this secondary in- 

 crease in thickness has begun, chloroplasts are present. (Fig. 

 6.) Relatively few are found in the central portion of the pith, 

 but the chloroplasts become numerous in those cells lying at the 

 outer edge of the pith area. 



The Medullayy Rays. 

 The medullary rays vary in different parts of the stem in 

 frequency of occurrence and in extent of tissue. They are 

 found in that part of the stem where the vascular system is 

 made up of comparatively distinct, isolated bundles, but be- 

 come inconspicuous and tend to disappear as the vascular 

 bundles approach the position of a complete ring. In the very 

 young stem chloroplasts are found scattered here and there in 

 the medullary ray cells, showing that here, as in the pith, 

 there is some photosynthetic action. 



The Xylem. 



The xylem of the stem varies from isolated bundles to a 

 complete cylinder or ring around the stem. It is made up of 

 wood parenchyma, wood fibers, fiber tracheids, and tracheal 

 tubes. The wood fibers, in company with the tracheal tubes, 

 make up the larger part of the xylem tissue. The fiber 

 tracheids are fairly plentiful, and are of various lengths and 

 widths, but are always much shorter than the wood fibers. 

 The water tubes average 12 to 14 in the region of each vas- 

 cular bundle. Along the tube, at intervals which average .058 

 mm. in length, appear the remains of the original cross-walls 

 at the ends of tube elements, as they seem in some cases to lie 

 in the plane of union of adjacent tubes. They are shown in 

 figure 51. 



The Phloem. 



The phloem region, as that of the xylem, is variable in ex- 

 tent, being found as isolated portions, and also as a continuous 

 ring, undivided even by medullary rays. The phloem is com- 

 posed of extremely small cells in cross section, these being 

 phloem parenchyma, sieve tubes, and companion cells. The 

 sieve plates are thin and have perforations so minute as to be 

 clearly distinguished only by the aid of the 2 mm. oil immer- 

 sion lens. In the phloem cells are found stored nitrogenous 

 substances. 



