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number is increased often to six or seven layers. In regard to 

 these collenchymatic layers being necessary to support the stem, 

 it may be mentioned in passing that this stem is one of the class 

 having no bast fibers in its outer or rind portion. 



The real periderm consists of the wings until the stem gets 

 older — two or three years in some instances — when the assimilat- 

 ing surface is no longer needed, the bands of phcllogen cells ex- 

 tend themselves until a circle is formed around the entire stem. 



Now it is with the stem before the wings form and for some 

 time after this, that we have to do. In order to allow the cells 

 under the epidermis to fulfill their office of assimilating, the epi- 

 dermal cells themselves must remain intact and perform their of- 

 fice as do the epidermal cells of the leaf. It is necessary, there- 

 fore, that they divide and increase rapidly in order to keep pace 

 with the increasing growth in diameter. This in itself presents 

 no difficulty as long as the cell contents are in working order and 

 the cell wall is of pure cellulose, so that it can change in any way 

 to enable it to increase in thickness or in surface ; this process of 

 growth and cell division is normal to the plant. 



There is, however, one difficulty in this for the epidermal 

 cells, which is not found usually elsewhere. This is the heavy 

 upper or outside wall, the cellulose of which turns to subcrin or 

 cutin at a very early stage of the growth of the stem. One of 

 the characteristics of suberin is that it is nearly impervious to 

 air. It is also supposed to be much less lieible to change of 

 structure than cellulose. By this is meant that the micellae com- 

 posing it do not so readily change place, therefore change in 

 form or structure is more difficult in case of suberized wall than 

 that consisting of pure cellulose. In other words a cell with 

 purely cellulose wall with living contents is always liable to 

 changes by means of growth, while the wall turned to suberin or 

 lignin is a sign of completed growth, fixedness of form. 



Now in our Euonymits stem at a distance of one-half centi- 

 meter from the tip these epidermal cells are found partly suber- 

 ized as to their upper wall, this wall is thicker than the other, but 

 tlie shape of the cell is nearly cubical. At a distance of three 

 centimeters from the tip and from this on till the stein has ac- 

 quired some age, the outer walls of the epidermal cells are very 



