CLADODE OF RUSCUS 



159 



Several of these features can be observed in a cross-section of 

 the cladode of the Butcher's Broom {Riiscits aculeatus, Fig. 80). 

 The epidermis {cp.) has thick outer walls furnished with a pro- 

 nounced cuticle, but there is no appreciable depression of the 

 stomata. The latter [St.] are protected by the development, on 

 the external portions of the contiguous faces of the guard-cells, 

 of two pairs of ridges which extend upwards towards the pore ; 

 the outer pair are the more prominent, whilst the smaller inner 



Fig. 80. — Transverse section through part of a cladode of the Butcher's 

 Broom (Riiscus aculeatus), showing a single stoma (St.), the thick- 

 walled epidermis (ep.), and the aqueous tissue (Aq.) below the assimi- 

 latory zone. The chloroplasts are shown black. 



ones, which correspond to those present in most stomata, almost 

 meet in the middle line. In consequence two antechambers are 

 interposed between the actual pore of the stoma and the outside 

 air, the larger and outer being known as the vestibule. The upper 

 and lower epidermis are alike as regards thickening of the walls 

 and distribution of stomata, and this, as well as the absence 

 of a marked pahsade layer, can be related to the " edge-on " 

 position which this modilied branch assumes. The assimilatory 

 tissue, which is almost uniformly developed, exhibits rounded 



