44 CACT.E, OR PRICKLY PEARS. 



CMjiitaiii the .s])ha'roraphi(les of oxalate of lime, and .sIkjw 

 like silver stars under ))olarise<l light. The cells of the 

 <lei'inis increase in size in each successive and deeper layer. 

 In some places there is a sixth layer, whose cells deviate 

 from tlie usual l)rick-shai)ed form and are cut off at the 

 corners so as to l^e aliuost prismatic. This change fiom 

 the brick-shaped to the prismatic form can 1)e gi'adually 

 traced from the surface inwards. 'J'he dermal cells ai'e 

 verv thick walled, and the cell walls take un the stains as 

 "freelv almost as the nuclei of the cells. 



Since the flattened joints of Opuntia have to perform 

 the functions both of leaf and stem, it is not surprising 

 to find the cortical tissue i)laying the part, and copying 

 the structure of the outer mesophyll of foliage leaves, and 

 the same term, ])alisade parenchyma, may be a})plied to it 

 This tissue is found immediately below the dermis, on 

 ^ach side of the stem, with a depth of about 10 rows of 

 oblong cells, set with the long axis at right angles to the 

 epidermis. An average cell is about ,'2 mm. long by about 

 half that breadth. The outer palisade cells are supplied 

 with numerous chloroplasts, but these are replaced J)y 

 leucoplasts in the inner layers. When viewed in direct 

 sunlight, or in polarised light, the walls of these cells were 

 seen to contain minute crystals of acicular form. Some 

 few cells of the center or sub-dermal rows contained niasses 

 of yellow brown crystal.'-', the so-called crystal sand. I'he 

 fiphferocrvstals were J^j mm. in diameter. See Plate III., 

 Fig. 1., and Fig. 2. 



[I —Opiintia inerniiv, D.C. 



The same general structure is seen in this as in 

 O. uHinntiaca, but the sphere-cry.stals though quite as 

 nu)nerous, and occupying the .same sub-epidermal position, 

 are smaller, varying from -\, to ^'„ mm. The \\alls of the 

 palisade cells are studded with smaller and isolated 

 crvstals. Tawny masses of crystal sand were also evident 

 in this sjjecies. Plate IV.. Figs. 1 and 2. 



Ill a transverse section of a joint or flattened stem, 

 the vascular bundles are found at the base of each mass 

 ■of palisade parenchyma. The wood vessels are few in 



