428 



dampness with a continued excessive supply of water at the roots and a 

 decreasing intensity of hght. An insight into the production of these phe- 

 nomena may be found in the 



Cork Disease or the Cacti. 



This disease, often found in imported cacti, has become a constant 

 source of anxiety for the European grower. It manifests itself in the 

 different varieties of cactus, in the appearance of dry, papery places. These 

 begin sometimes as raised yellow spots, or as spots remaining green and 

 looking somewhat glassy. They widen out cither into large cork colored 

 surfaces, or become depressions which look like the scars of places injured 

 by biting insects or animals. My special studies were first of all with 

 Cereus flagelliformis. In severe cases the tips of the stems still seemed 



Fig. 70. Piece of the trunk of a I'hyllooactus which, on its under side, exhibits 



cork excrescences in the form of warts, while, on the opposite side, the 



process of i)erforation is beRinninff. 



fresh and green, but at a little distance back from the tip a zone of rust 

 colored specks began, starting usually below a thorn cushion. The specks 

 gradually united into a rusty surface which ruptured here and there. 



On the healthy part, the outer epidermal tissue consisted of two layers 

 of irregularly 4 to 6 sided cells wuth thickened, heavily cutinized outer walls. 

 Under this double layer was a single row of cells elongated tangentially and 

 thickened like collenchyma. Then came the bark tissue containing chloro- 

 phyll and an abundance of crystals of calcium oxalate. Cork had been 

 formed in the outer epidermal cells of the rust spots on the stems. The 

 cork cells were wall-like in some places, irregular in others, like a cap which 

 finally ruptured on the crest, thus rupturing the outer w^all of the upper 

 epidermal layer. 



In other Cereus species, different sides of the stem seemed whitish and 

 dry in wide stretches. Here cork layers formed in the epidermal cells in 

 the angle of the stem; these were raised like papillae, while on the surface 



