HISTOLOGY OF STEM OF THE WAX PLANT 267 



as well as in the stem, and being so abundant, their development can 

 easily be traced, and the changes that take place in the contents of 

 the cells that later become transformed offer an interesting field 

 for investigation. It is difficult to account for their presence. The 

 sclerotic girdle may be readily supposed to be for the purpose of 

 strengthening the stem, but these separate cells cannot be considered 

 in that light, as they are surrounded by parenchyma and simply add 

 their weight to the stem without contributing to its support. They 

 do not connect with each other except where two or more happen to 

 be in contact, and therefore can hardly be considered as part of the 

 system of the plant. They may better be regarded as the result of 

 secretions for which the plant has no further use. 



Sclerotic Girdle 



The sclerotic girdle makes its first appearance in the third or fourth 

 internode by the lignification of some of the cells of the parenchyma 

 surrounding the bast, thus forming a broken ring. The intervening 

 cells soon become lignified, and we have the girdle formed of a single 

 row of cells which afterward increases to two or more rows. 



Now if a young portion of the stem in which the sclerotic girdle 

 has not yet formed is treated with iodine, the first row of cells sur- 

 rounding the bast will be seen to be filled with starch forming a distinct 

 and striking circle of a dark blue color. It is the row of cells surround- 

 ing those containing the starch that develop into the sclerotic girdle, 

 and it seems almost impossible to resist the conclusion that the starch 

 is in some way connected with the growth of the sclerotic cells. 



Lignification of the Sclerotic Girdle 



The walls of the cells that form the girdle are at first slightly 

 thickened on all sides, leaving small pores which later form canals 

 as shown on pL II, fig. 7. I have not been able to decide whether 

 these ''pores" are holes through the cells or spaces left unlignified^ 

 but I think the latter, as it seems that if they were holes they would 

 appear before lignification, which, as far as my knowledge extends, is 

 not the case. The cell walls then increase in thickness either by the 

 formation of substance on the inside or interfiltration, between the 

 pores, which thus become canals. In my opinion the cell walls are 

 formed by layers added to the inside, as they are distinctly striated. 



