44 



tention. In other specimens, the epidermal cells when elongated lie in 

 a parallel direction with the line of outer surface. In this case, they are 

 elongated at right angles to the outer surface. Their thread-like walls 

 are contiguous with the cuticle on the outside ; while, on the inside, they 

 are bounded by a single hypodermal row. Their only apparent outer 

 wall is # the thickened cuticle. 



The hypodermal regions seen may be at once divided into two classes. 

 Those of the hrst and larger class may be characterized as follows : cells 

 irregular, in several layers: walls thick, pitted, collenchymatous. The 

 second class, on the other hand, are thin-walled, regular, and disposed in 

 one layer. Six Mamillarise and five Cerei will fall under this latter class. 

 The highest number of collenchyma layers is nine, found in Cerent grandi- 

 flora. The number of rows, shape of cells and relative thickness of walls 

 appear to be constant within the limits of species, and may be of service 

 as determinative characters. 



It remains only to mention the calcium oxalate crystals, which are of- 

 ten distributed as constituents of the cell-contents, both in the epidermal 

 and in the hypodermal tissue. These occur in the form of simple, soli- 

 tary, klino-rhombic crystals, or more frequently in angular, stellate 

 groups. In size, form and position they vary exceedingly, but appear 

 uniform within the limits of the same species. Crystals occurred in every 

 Opuntia and in every Kchinocactus examined. In Mamillaria they were 

 frequent, while in Cereus, they were with one exception entirely wanting. 



The minute structure of the spines is exasperatingly uniform. The 

 outer, or epidermal cells are usually large and thin-walled, while in the 

 body of the spine the walls are so thick as to entirely close the cell cavi- 

 ties, as is the case in all dense woody tissues. Often there is a gradual 

 transition from one to the other. The important characters are in the 

 outer row of cells. Iiou.^h projections partaking of the nature of tri- 

 chomes, and extending toward the spine tip are common in Cereus and 

 Echinocactus. In Mamillaria the spines are smooth or rarely pubescent, 

 as in M.pusiila, Grahami and allied forms. Those of the cylindrical and 

 clavate groups of Opuntia are without exception clothed with a semi- 

 transparent, glistening sheath ; while those of the flat-jointed Opuntias 

 are naked. Characteristic of Opuntia spines is the ronical arrangement 

 of libers, distinctly seen with the low power objective. Spine fibers of 

 other genera are usually parallel from base to tip, whereas here they are 

 conically arranged with the summits of the cones at the extreme tips and 



