SECONDARY XYLEM 117 



The periphery of flu pith is formed of several layers of 

 rather thick-walled more or less isodiametric parenchyma cells 

 which are provided with protoplasts and in addition contain very 

 small starch grains; other of these peripheral cells are filled with 

 dark-staining granular material, the exact nature of which is ob- 

 scure at present. The bulk of the pith is composed of large 

 "isodiametric" parenchyma cells which are separated by definite 

 intercellular air-spaces ; some of these cells likewise contain small 

 starch grains and protoplasts. Interspersed among these large 

 pith cells occur much smaller thicker-walled parenchyma cells 

 which are filled with granular dark-staining bodies ; these latter 

 cells are found singly or in groups, but in longi-section appear in 

 scattered vertical series. 



Attention must finally be directed to the mucilage-containing 

 cells which are arranged in a more or less definite ring near the 

 periphery of the pith. These rnucilage -containing cells may be 

 identified by their large size, by the disorganized reddish or purple 

 material found in them, and by the jacket of starch-containing 

 parenchyma cells which surrounds each of them. The function 

 of the mucilage formed in these cells is unknown. 



3. The stem of Gymnosperms (e.g., conifers) is similar to that 

 of many woody dicotyledons in that the activity of a cambium 

 forms a cylinder of secondary phloem centrifugally and a cylin- 

 der of secondary xylem centripetally. A transverse section of a 

 young pine stem shows a number of interesting anatomical fea- 

 tures, viz. : 



(a) The large cavities or resin canals which appear in the 

 cortex and in the secondary xylem (in the latter region the canals 

 are smaller than those in the cortex). Notice that the resin canal 

 (which has arisen by the pulling apart of groups of cells in certain 

 regions and is hence of the schizogenous type) is bordered by a 

 ring of small, densely protoplasmic secretory cells which exude 

 resinous material into the canal. In certain species of pine, the 

 resin is of great commercial importance and is obtained by "tap- 

 ing" the trees; the resin obtained in this way is the basis of tur- 

 pentine and allied products. 



(&) The secondary xylem is relatively simple in structure and 

 consists of tracheids, fiber tracheids and uniseriate xylem rays. A 



