SPERMAPH YTES : PH^NOGAMS. 137 



and gradually assume the character of the cells on which they border 

 (Fig. 76, B, cam.}. The growth on the inside of the ring is more rapid 

 than on the outer border, and the ring continues comparatively near the 

 surface of the stem (Fig. 76, A, cam.}. The spaces between the bundles 

 do not increase materially in breadth, and as the bundles increase in size 

 become in comparison very small, appearing in older stems as mere lines 

 between the solid masses of wood that make up the inner portion of the 

 bundles. These are the primary medullary rays, and connect the pith 

 in the centre of the stem with the bark. Later, similar plates of cells are 

 formed, often only a single cell thick, and appearing when seen in cross- 

 section as a single row of elongated cells (O, TO). 



As the stem increases in diameter the bundles become broader and 

 broader toward the outside, and taper to a point toward the centre, appear- 

 ing wedge-shaped, the inner ends projecting into the pith. The outer limits 

 of the bundles are not nearly so distinct, and it is not easy to tell when 

 the phloem of the bundles ends and the ground tissue of the bark begins. 



A careful examination of a cross-section of the bark shows first, if 

 taken from a branch not more than two or three years old, the epidermis 

 composed of cells not unlike those of the leaf, but whose walls are usually 

 browner. Underneath are cells with brownish walls, and often more or 

 less dry and dead. These cells give the brown color to the bark, and 

 later both epidermis and outer ground tissue become entirely dead and 

 disappear. The bulk of the ground tissue is made up of rather large, 

 loose cells, the outer ones containing a good deal of chlorophyll. Here 

 and there are large resin ducts (Fig. 76, H], appearing in cross-section as 

 oval openings surrounded by several concentric rows of cells, the inner- 

 most smaller and with denser contents. These secrete the resin that 

 fills the duct and oozes out when the stem is cut. All of the cells of the 

 bark contain more or less starch. 



The phloem, when strongly magnified, is seen to be made up of cells 

 arranged in nearly regular radiating rows. Their walls are not very thick 

 and the cells are usually somewhat flattened in a radial direction. 



Some of the cells are larger than the others, and these are found to be, 

 when examined in longitudinal section, sieve tubes (Fig. 76, E) with 

 numerous lateral sieve plates quite similar to those found in the stems 

 of ferns. 



The growing tissue (cambium), separating the phloem from the wood, 

 is made up of cells quite like those of the phloem, into which they insen- 

 sibly merge, except that their walls are much thinner, as is always the 

 case with rapidly growing cells. These cells ( Z?, cam. ) are arranged in 

 radial rows and divide, mainly by walls, at right angles to the radii of 



