HISTOLOGY OF STEM OF THE WAX PLANT 273 



They have rather thick lignified walls that are marked by pits having 

 a slit-like appearance, the longer axis of the slit forming an angle 

 with the sides of the cells. 



Medullary Rays 



Tie cells of the medullairy rays in the plant we are considering 

 present an exception to' those usually found, in that they stand verti- 

 cally, or with their greatest length parallel to the axis of the stem. 

 Generally these tissues are well marked in a tangential section by their 

 lenticular form and the difference in the thickness and composition of 

 the walls of their cells as compared with those of the other components 

 of the wood ; they show equally well in a radial section the fact that 

 their greatest length is along the radial line, thus crossing the axis of 

 the other cells at right angles. 



In Hoya, as stated before, tangential sections of the old parts of 

 the stem show lenticular masses of parenchynia that might be easily 

 mistaken for medullary rays. The young stem, however, does not 

 contain them, and if we admit they are the rays, we must concede that 

 none exist in the younger portions of the plant. If a radial section of 

 a stem is examined, cells will be noticed that are quite regular in shape, 

 as shown on pi. I, fig. 6, being prismatic in form, and, as shown in 

 the section, rectangular in outline and usually quite filled with starch. 

 Their sides are pitted, as are also the ends. They are generally about 

 I mm. in length, though sometimes longer or shorter, and about 

 .0.15 mm. in diameter. In a cross-section of the stem they may be 

 easily recognized by the starch they contain and by the pitted plate or 

 end of the cells, some of which are generally shown. " But few minute 

 investigations on the structure of the cells of medullary rays exist, and 

 many details of structure are still to be discovered." ' 



If I am correct in supposing the tissues described above as medul- 

 lary rays, they resemble the wood cells very closely in all but their 

 shape. They run from pith to bark a,nd are marked with pits similar 

 to those of the wood cells. The inner boundary of the wood is formed 

 by a few long cells with the walls thickened in a spiral or annular 

 manner. In cutting a section the cells separate very readily and, as it 

 were, uncoil. They are lignified like the wood, but in cross-section 

 are circular with the walls slightly thinner. 



' De Bary, "Comparative Anatomy of Ferns and Phanerogams,''- p. 486. 



