296 FKEDERICK A. WOLF 



terposed in layers within thin-walled cork-tissue, is not collen- 

 chyma but cork. Neither is it collenchyma tissue which forms 

 the callous layer in case of wounds arising from perforations 

 either by anthracnose or mechanical injuries. To the peculiar 

 kind of cork which is formed in these cases the name stone cork 

 is applied. If one attempts to recognize all cork tissue micro- 

 scopically after having formulated his ideas of its nature from 

 the accounts and illustrations in many of our text-books, he will 

 probably be in error. • Cork may not only consist of tabular thin- 

 walled cells but the walls may be more or less modified by sec- 

 ondary thickening. This secondary thickening may in some 

 cases, as in the beech, be quite uniform. On the other hand the 

 outer wall may be much more strongly modified as in the willow 

 and pear and as is also the case in Opuntia lindheimeri. Neither 

 the willow nor the pear, however, have such strongly thickened 

 tissue as the cactus, neither do their cork cells have pitted walls. 

 In the case of Viburnum opulus the inner wall has the greater 

 thickness. 



Again, a microchemical test alone will not furnish conclusive 

 proof. Frequently cork tissue will be found which is suberized 

 throughout its entire thickness, as shown on treatment with a 

 solution of alkanet. Mohl 5 has shown that in Boswellia papyri- 

 fera the cork tissues are silicified. Haberlandt, 5 because of the 

 chemical reactions with proper reagents, concludes that cellulose 

 and ligneous substances may in some cases be present. The 

 presence of cellulose in cork tissue was confirmed by the later 

 researches of Gilson and van Wisselingh. 5 Because of the fact 

 that the stone cork cells of cacti yield so readily the characteristic 

 reactions for cellulose, I was led to believe that they were collen- 

 chymatous. When, however, one takes into account the origin 

 of this tissue, he is presented with convincing evidence of its 

 nature. Because of the fact that the stone cork cells are arranged 

 seriatim just as are the adjacent thin-walled cork cells, and that 

 several layers may be formed, separated by thin-walled cork 

 tissue (fig. 9), it is very evident that both kinds of cork cells arise 



5 Vide DeBary's Comparative Anatomy of the Phanerogams and Ferns, 1884. 

 and Haberlandt's Physiologische Pfianzenanatomie, 1909. 



