BY JOHN SHIRLEY, D.SC, AND C. A. LAMBERT. 45 



number*, and u.sually .showed spiral strengtheniiiii layers. 

 The bast vessels form a larger mass of tissue, and in a few 

 of the vascular bundles appeared to l)e bi -collateral. 



Jn the palisade parenchyma, and also in the meduUarx 

 or water-storing tissue of the stele, are numerous air- 

 • cavities. The inner walls of these cavities take up some 

 stains readily, and, under a low power, look like dark 

 blotches, but with proper focussing, and with a high power, 

 the outlines of the cells lining them can be revealed. In 

 a rather thick section, stained with hsematoxylin. there 

 are signs of tubes connecting the air cells of the medulla 

 with those of the two layers of parenchyma, and in all 

 cases there are passages from the stomata to the air-spaces. 

 The breathing spaces are I to ^V mm. in greatest diameter, 

 and are usually globular or elliptical in shape. Plate V.. 

 Fig. 1. 



On each sitle of the vascular bundles the s])here-erystal.s 

 are thickly scattered ; they are sparingly seen in the 

 walls of the medullary tissue or aqueous tissue as it has 

 been termed, because its cells store uj) water after every 

 fall of rain.f 

 III. — Opuntia monacantha. Haw. 



An examination of stems of this pear make it easy 

 to understand why the cochineal insect can damage thi.s 

 species, while its attacks have no effect on such plants 

 as 0. inermis and 0. auranliaca. The armour of sphere- 

 crystals is almost absent from the subepiflermal layer 

 of the dermis, and though there are traces of small oxalate 

 of lime crystals in the walls of the palisade cells, they are 

 only seen with high powers, and are not by any means too 

 evident even then, though sought for with the aid of 

 polarised light. In all other respects the histology agree* 

 with that reported for the species already mentioned. 



IV. — Peireskia aculeata, Mill. 



The Barbadoes Gooseberry possesses true foliage 

 leaves, and its flowers prove an excellent attraction for 

 bees ; specimens seen near the river baiaks at C'orinda 



*Possibly because for ease in sectioning young tissue wa.s nsually 

 selected. 



fKeiner and Oliver, I., 329. 



