HISTOLOGY OF STEM OF THE WAX PLANT 263 



to the axis of the stem. It is difficult to estimate the number in a given 

 space, as they are very irregularly distributed and the proportion 

 varies with the age of the stem. In a square millimeter of epidermis I 

 have counted ten stomata in one case and in others but one; perhaps 

 five would be a fair average for the younger portions of the stem. 



Papillae 



These are developed on the older portions of the stem and appear 

 to be formed by a thickening of the central portion of the outer wall 

 of the cells of the epidermis. 



Cork 



As stated before, the outer wall of the epidermis is deeply cutinized, 

 but in the younger portions of the stem we find no cork. As the stem 

 increases in age and the lateral walls of the epidermal cells become 

 suberized, the next cell, the cork cambium or phellogen, divides 

 tangent ially, the outer half forming the first cork cell which is suber- 

 ized in the same manner as the epidermis. The inner half of the cell 

 again divides, and in this way cork is continually formed. The 

 development of the cork is shown on pi. II, fig. 4, in which p is 

 the phellogen which divides to form the cork cells c. The alteration 

 that takes place in the cell walls changing them from cellulose to cork 

 proceeds from the outer wall to the inner one. 



"It is not possible to account satisfactorily for these substances, 

 subterin and lignin, in cell walls. They are probably the result of 

 modification of the cell wall and not of interfiltration." * This state- 

 ment is supported by the manner in which the phellogen cells of Hoya 

 camosa become suberized. Treatment with sulphuric acid shows 

 this very plainly. In the younger portions of the stem the epidermis 

 alone is left after the parenchyma is destroyed, then the lateral and 

 finally the inner wall remains, the suberization always taking place 

 toward the phellogen. Now it has been demonstrated that in coni- 

 fers the cells become suberized after they have lost their protoplasmic 

 contents, and this would seem to be the case in the cork of Hoya 

 camosa. 



As soon as the cork is formed, the cells on the outside are shut off 

 from further supply of nourishment, as far as we know, and if the 



1 " Physiology of Plants," S. H. Vines, p. iS. 



