132 PLANT HABITS AND HABITATS IN THE 



There is a distinct bifacial structure of the leaves of this species 

 of Pittosporum. The stomata are confined to the ventral side. The 

 chlorenchyma of the dorsal side is more consistently of palisade cells 

 than is that of the other. The stomata are on a level with the epi- 

 dermis. Crystals are fairly numerous in the mesophyll. A relatively 

 small amount of sclerenchyma, not well developed, is to be found in 

 connection with the main conductive tracts and on the leaf-edges. 

 But the most striking feature of the structure of the leaves is the 

 presence of an epidermis on the dorsal side, consisting of more than 

 one cell-layer. The outer epidermal cells are approximately one-fifth 

 as deep as the underlying epidermal cells, and these are occasionally 

 accompanied, as shown in figure 29, by narrow cells which appear to 

 be situated in their outer walls. The two-celled condition of the 

 epidermis continues around the edges of the leaves, and a few epidermal 

 cells on the ventral side may be of two layers. On both ventral and 

 dorsal surfaces the outer epidermal wall is very thick, and the inner 

 walls of the epidermis on both leaf-sides are also somewhat thickened 

 (fig. 30). 



Triodia irritans. 



The occurrence of Triodia throughout the driest portion of the in- 

 terior of the continent, characteristically in sandy habitats, makes the 

 structure of especial interest. T. irritans is a "bunch" grass, with 

 fairly rigid, sharp-pointed leaves. It is sometimes called "porcupine" 

 grass, which aptly suggests its nature. 



A cross-section of a leaf of Triodia is nearly circular in outline. 

 This is due to the in-rolling by which the ventral surface comes to lie 

 within, and only the dorsal surface is presented to the outside (fig. 31). 



On the dorsal outer as well as the inner surfaces are several deep 

 furrows, giving the leaf-surface a convoluted effect. The chloren- 

 chyma is segregated on either side of the furrows, so that in section 

 the chlorophyll-bearing tissue occurs in separate masses. Scleren- 

 chyma constitutes a very prominent part of the tissues of the leaf. 

 On the dorsal side it reaches to the epidermis and extends to about the 

 depth attained by the chlorenchyma, and on the ventral inner side it 

 also reaches to the epidermis but does not occupy much of the spaces 

 between masses of chlorenchyma, which is filled in with parenchyma 

 having faiily heavy walls. The outer epidermal wall is heaviest in 

 the furrows, but is not especially heavy elsewhere. Trichomes are 

 present sparingly on the in-rolled surface and short hairs close the 

 narrowed "throat" of the fuirows. Stomata are small and confined 

 to the portion of the furrows opposite the chlorophyll-bearing cells, 

 and are present, but not in abundance, on both leaf-surfaces. 



