44 PROCEEDINGS OP THE ROYAL SOCIETY OF QUEENSLAND. 



triangular in outline, the epidermis is well shown, the outer 

 walls so strengthened with cutin as to form half the radial 

 diameter of each cell. A circle of sclerenchyma lies between 

 the phelloderm and the phloem, lying in a series of curves round 

 the vascular bundles, with larger thick- walled cells where two 

 curves meet, some of which have the diameter of vessels. In 

 older sections of 4-5 years, the vessels have their greatest dia- 

 meter parallel to the circumference, and the lines of wood-cells 

 are nmch distorted around vessels and near the outer portions 

 of the xylem. The pith-cells are full of starch grains, and a 

 few contain tabulate crystals. In tangential longitudinal 

 section both aulacocarpd and cunninghamii show the cells of 

 the medullary rays in uniseriate arrangement ; in the former 

 the ray sections are 210-225 /x by 9-12 /x ; in the latter 

 110-210 ju. by 10 /x. A. cinci7inafa shows alternating layers of 

 hard and soft bast in the phloem, but otherwise its histology 

 is that already described for aulacocarpa. 



In cunninghamii the joith cells are large, filled with starch, 

 often elongated in a radial direction and bounded by a wavy 

 outline of pro to xylem. The primary medullary rays are 

 strongly marked, and are formed of cells of considerable 

 radial diameter. 



Neither endodermis nor pericycle is clearly shown in 

 any member of this group. 



IV. — Acacia penninervis Sieb., in transverse section, is re- 

 markable for the numerous broken concentric rings of 

 sclerenchyma or hard bast in the phloem, and for the thickness 

 of the bast layer. The vessels vary in diameter from 43 [m to 

 105 fx. The wood-cells produced in autumn have their walls 

 much thickened, and each annual zone is thus made to add to 

 the strength and elasticity of the stem. Dotted tracheides are 

 more than usually numerous. 



Passing through the phloem, the cells of the medullary 

 rays remain uniseriate, but acquire a greater transverse 

 diameter, and are plainly seen in the more internal masses 

 of sclerenchyma. In tangential section they measure 103-220 [jl 

 by 6-17 [X. In the phelloderm, at intervals, the large cells are 

 seen to be placed with the long axis directed radially ; in 

 other parts of the inner cortex the longitudinal axis of each cell 

 of this inner layer is vertical. These cells retain their proto- 

 plasm, and have a rather narrowly elliptical nucleus. (Plate 

 VI.) 



