156 FRANCIS E. LLOYD AND CHARLES S. RIDGWAY 



NOTE ON THE DEVELOPMENT OF TRICHOMES'AND AREOLAR CORK 



The floor of the sulcus or areole from which the nectar gland 

 arises is thickly clothed with trichomes, each consisting of a single 

 row of cells. They are very long, of uniform thickness, and increase 

 their length by the elongation of new cells produced basipetally. 

 When however the trichomes cease to increase in length, the epider- 

 mal cells from which they arise continue to divide (fig. 7) below 

 the level of their outer walls, this forming a periderm-like layer 

 of cells, which are more or less cutinized. Similar divisions, par- 

 allel to those in the epidermis then appear in the underlying 

 parenchyma (fig. 8), or, as it may be in some species, collenchyma, 

 adding more, but not uniformly or completely suberized, tissue 

 to the imperfect cork already formed by the epidermis. There 

 may thus be developed a corky pad of considerable thickness 

 (fig. 9, c), consisting of very numerous thin-walled, collapsed and 

 imperfectly suberized cells forming the floor of the areole. 



The impression is created, by this peculiar mode of develop- 

 ment, of concrescence of the trichomes as they become older at 

 the basal portions. The proper interpretation appears to be that, 

 when the trichomes reach the limit of development, the epidermis, 

 which has contributed to the trichome development, now forms 

 a cork layer, this being increased in thickness by the removal of 

 the periderm divisions to the underlying tissue. Figs. 7 and 8, 

 which were taken from the edge of the trichome bearing cork pad 

 in a species of Mamillaria 15 show clearly that the epidermis is 

 involved in cork formation 16 as first noted by Schleiden. 17 



15 No. 31, Lloyd, Cacti of Northern Zacatecas. National Herbarium. 



16 F. A. Wolf has observed a similar behavior in the formations of cork islands 

 in Opuntia lindheimeri. Ann. Myc. 10:113-134, 1912. 



"Through Solereder, Sys. Anat. d. Dicot. 



