Cold-Resistance; in Spineless Cacti 123 



HISTOLOGICAIv STUDIEvS 



GENERAL STRUCTURE OP THE OUTER, THICKENED PART OR 

 INTEGUMENT OE A CACTUS STEM 



It is quite difficult to make thin hand sections of living cacti 

 and similar succulent plants, as the tissue is soft and often slimy 

 and does not section readily. It was found that sections could 

 be made for ordinary study with a hand microtome. In order to 

 study the tissues from a histological or cytological standpoint, 

 however, it was necessary to embed small pieces of the cactus in 

 paraffin and make sections with a rotary microtome. 



A histological study of certain parts of the cactus plant is- 

 very essential, as it is one of the means which may determine- 

 whether a certain species is hardy or not. The difference between: 

 the outer tissue layers of different species of cacti, especially those 

 which are resistant to certain low temperatures, and those which are 

 not, is very remarkable. In order to show these differences, joints 

 oi several species of Opuntia of different ages were selected. 



I first made a study of the layers of tissue outside the woody 

 bundles of a well developed two-year-old joint of Opuntia castillae. 

 In studying a section of this under the microscope, the thick-mem- 

 braned cells at the periphery, or outside, and the large thin-walled 

 cells toward the center of the stem, or joint, were observed. 



The first layer encountered is the epidermis; it can still be detected 

 easily in stems which are three and even four years old. The 

 epidermal cells of two-year-old stems sometimes contain protoplasm; 

 however, this seems to be an exception in the plants which were 

 worked with. The epidermis soon secretes a cuticle or secondary 

 membrane, which after a few months becomes quite thick. When 

 young this is stained light yellow with a concentrated solution of 

 potassium hydroxid; older cuticles stain darker yellow, which sug- 

 gests that the outside membrane of the epidermis is heavily satur- 

 ated with suberin, the same substance which forms in cork. The 

 stomata, or breathing pores, are small but numerous and are 

 embedded in the cuticle. 



Immediately below the epidermis follows a layer which we will 

 call the "crystal-bearing layer," since its cells always bear crystals. 

 Its outer and lateral membranes are thin, but the ones toward the 



