LEAVES OF IIAKEA. 



285 



the assimilative tissue, and have root-hke extensions at right 

 angles at both ends. Each idioblast cell rests by means of these' 

 extensions against the epideraiis on one side and against the 

 central cylinder on the other (Fig. 1). Obviously these cells are 



Fiji- 1. — Tran.sver.se Section of Leaf of lJ<iLi(i jnctiiKitti. 



mechanical in function, and by their means tiie sh-ipe and I'igidity 

 of the cylindrical segments are maintained. Especially do' they 

 })revent collapse of the tissues. This leaf has probably evolved 

 on xerophytic lines from a flat, more expanded and delicate type. 

 These idioblast cells do not constitute a xerophytic character in 

 themselves, but by their means the leaf was enabled to as.sume, 

 as it were, the cylindrical form. 



In the case of Hal'va suavcolenf<, a common hedge plant of 

 the Transvaal, the leaf is simple, thin, flattened, ratht:>r narrow 

 and bifacial. It still, however, shows xerophytic characters, such 

 as toughness, thick cuticle, and sunken stomata. In section, 

 similar idioblast cells are found to be present in large numbers, of 

 exactly the same type as those found in H. pccfinata. Each 



Fig. 2. — Transverse Section of Leaf of Tlal^va ,sun n'uhiis. 



extends, however, in this case from eithei" the upper or lower 

 epidermis to- the middle of the leaf, where the extensions at the 

 extremities either intermingle or rest against some of the 

 mesophyll cells. None of them passes across the leaf from one 

 epidermis to the other (Fig. 2). The use of such idioblast cells 



