

1920] 



BROWN— STOMA TA 



297 



but for convenience its main features will be given here. 



It is a 



small room situated under the floor of the main laboratory, from 

 which light is absolutely excluded by means of an antechamber 

 and two doors, the second door being a trapdoor in the ceiling of 

 the dark chamber. The temperature remained between 56 and 

 75° F., with a maximum weekly fluctuation of 2 F. The humidity 

 was 80-90 per cent. The room was absolutely dark at all times 

 excepting for the candlelight used while collecting material. 



Some of the etiolated shoots were killed in the dark chamber, 

 others were removed to the light and allowed to grow for varying 

 lengths of time before they were killed. The etiolated shoots 

 removed to the light are referred to as etiolated-greened shoots. 

 The killing agents used were chromo-acetic acid and Bensley's 

 mercuric- formalin solution (1), and the material, both normal 

 and etiolated, was then prepared for sectioning in the usual way. 



Land's 



Several stains were used, including safranin-haematoxylon, orange 

 G, and Haidenhain's iron-alum. Because of the mucilaginous 

 nature of cactus material, it was found advantageous to continue 

 the washing operation after the killing agent somewhat longer 

 than is usually necessary with ordinary plant tissue. 



(3) fixative proved useful in handling large sections. 



Normal shoot 



The gross structure of the normal shoot has already been 

 given under the general description of the plant. Cross-sections 



(%. 



tegumentary 



spongy 



Exter- 



nal to the phloem there appeared a mucilage mass which, with 

 the bundle, presented a nail-shaped outline. The integumentary 



region consisted of 



rm 



(fig. 11). 



The cuticle averaged about 11 /x in thickness. Epidermal cells 



measured 



was about 45 /*. 



Opuntia 



other cacti observed and mentioned by Schleiden (6) is the 

 patchy decortication, in which the epidermal cells become active 

 in spots, divide periclinally, and the inner of the two layers of 

 cells thus formed continues to divide until a layer of tissue several 





