4t) CACT.I, OR PRICKLY PEARS. 



aiul in the Botanic Ciardens always claim attentif)n during 

 the floweiinji period by the buzzing of their numerous 

 visitors. 



The .se]>al.s have au upper and a h)wer epidermis, and. 

 as is usual, show little differentiation of the mesoi^hyll 

 into spongy and palisade parenchyma. Air sjjaces are, 

 very numerous, and take up a large portion of the whole 

 space filled by the mesophyll. Massed crystals, less acute 

 than tho.se forming the s])here-ci'vstals of Opuntia, are 

 found in the subepidermal layer, parallel to both surfaces. 



The stem shows the usual divi.sion, on transverse 

 section, into palisade layers, vascular bundles, and medulla 

 or aqueous layer, as in Opuntia : but in all tissues except 

 the vascular bundles, the cells are shortly oblong in outline, 

 the longer axis being to the transverse axis as 3 : 2. In 

 palisade cells the long axis is at right angles to the 

 epidermis, in aqueous tissue it is jjaiallel to the surface. 

 Plate VI.. Fig. 2. 



\.— Cfreus giandiflorus. Haw. 



Epidermal cells have the Mavy t)utline so common 

 in similar cells of fohage leaves in numerous dicotyledons. 

 The cells of the internal tissue are crowded with acicular 

 (true) ra})hides. or crystals of oxalate of lime ; and these 

 are seen like a halo round the circumference of the plant 

 section, having escaped from cut cells. A wound on 

 the surface of this cactus, exposes a number of slime- 

 <?ells containing raphides. Any rain or dew that fell would 

 be imbibed by them in excess, and, their walls rupturing, 

 the contained needles would be extruded, and form 

 <lefensive chfvaux-de-frise round the wounded portion. 

 Plate v.. Fig 2, and Plate VI., Fig. 1. 



YI. — Echinocactus Eyresii, Luc. 



The stem of this plant shows little differentiation of 

 internal tissue. The cells of the central medulla differ 

 little in shape from those of the paHsade parenchyma, 

 although the longest axis is parallel to the epidermis. In 

 places the true epidermis, which does not take up the stain, 

 is H-7 cells thick, the extra layers formed of cork cells. 

 It is ))r()bable that this is a defence against fungal attack. 

 On the opjiosite surface the epidermis was normal — one 



