ACACIA PHYLLODE 135 



bear a bipinnate lamina, is flattened in a vertical plane, 

 while in other cases where the petiole is larger, the 

 lamina is almost entirely absent : the latter is constantly 

 the case in A. armata. Note the reticulate venation. 



I. Cut transverse sections through a phyllode, mount 

 as before, and observe that the tissues are arranged 

 symmetrically with reference to the two flat surfaces : 

 thus 



1. Stomata are found in the epidermal layer of both 

 sides. 



2. Palisade - parenchyma is found immediately 

 below both epidermal layers. There may (A. hetero- 

 pJiylla), or may not (A. armata) be a central mass of 

 colourless tissue. 



3. Vascular bundles are found with their phloem 

 and sclerenchyma directed towards the outer surface, 

 and the xylem towards the centre : these bundles are 

 frequently opposite one another, and with their respective 

 masses of xylem in close proximity : compare the girder- 

 like arrangement of the vascular bundles and scleren- 

 chyma so common in the leaves of Monocotyledons. 

 Note also a single vascular bundle, strengthened with 

 sclerenchyma at both the upper and lower edges of 

 the compressed petiole. Compare the leaf of Iris 

 (see below, p. 179). 



II. An actual transition from the bifacial to the 

 isobilateral type of structure of the leaf may be traced 

 in Eucalyptus glolulus and other species, in which the 

 lamina of the lower leaves is extended in a horizontal 

 plane, the upper leaves of the plant in a vertical planes : 

 the former show two characteristic layers of palisade- 

 parenchyma below the upper epidermis, while the rest 



