HISTOLOGY OF STEM OF THE WAX PLANT 261 



containing the bast fibers which are scattered in bundles just within 

 the sclerotic girdle. 



6. The cambium composed of thin-walled cells. 



7. The wood containing the medullary rays, ducts, etc. 



8. The pith made up of parenchyma with intercellular spaces. 



9. A few sclerotic cells in the center of the pith similar to those 

 of the girdle, but in a section of a young stem, slightly larger, being 

 circular in outline; while those of the girdle are compressed, having 

 their shorter axis on the radial line. 



A section corresponding to the above description may serve as a 

 type with which to compare an older and a younger stem. In the 

 younger portion of the stem (the first internode) the most noticeable 

 features are the absence of the sclerotic girdle and cork cells, and 

 the increased number of hairs and stomata. 



In the older portions of the stem the principal changes consist in 

 the relative space occupied by the different tissues and their modifica- 

 tions. For example, in a section of a stem 10 mm. in diameter we find 

 the epidermis missing, and the cork, which is popularly known as 

 "bark," beginning to part in places. The space between the sclerotic 

 girdle and the cork has grown much narrower, and is dotted here and 

 there with many sclerotic cells like those of the pith. The sclerotic 

 girdle is composed of cells irregular in shape, and what we may call 

 their tangential diameter is now much greater than their diameter 

 measured on the radius of the stem. The ring of parenchyma con- 

 taining the bast has grown narrower and now contains sclerotic cells 

 here and there. 



The wood has increased and occupies a proportionately much 

 greater part of the stem. It is very irregular in outline and is often 

 penetrated by wedge-shaped masses of parenchyma. The ducts are 

 prominent owing to their increased number. The pith is contracted, 

 and in its center is a mass of sclerotic cells. Just within the ring of 

 wood will be noticed collections of small cells forming well-marked 

 patches around the pith. These I have been unable to find men- 

 tioned in any work at my command, unless they are what Davids, 

 in his "Ueber die Milchzellen der Euphorbiaceen, etc.," calls soft 

 bast bundles ("Weichbast bundel"). Crystals and milk cells will be 

 considered in treating the different rings of tissues. 



