07 
fire restricted to the shaded underside, and there, free circulation 
of air is prevented by a dense layer of short, curly hairs. These 
are present in such numbers as to give the lower surface of the 
leaf a white colour. Another xerophytic structure is the layer of 
mechanical fibres underlying the upper epidermis. These fibres 
remove the softer mesophyll tissues from the dry atmosphere and 
give a noticeably hard texture to the whole leaf. 
The reduction of the softer tissues from, which evaporation is 
liable to take place is characteristic of xerophytic plants. Their 
replacement by selerenchyraa renders the vegetation harsh and 
gives a profusion of spines- prickles and thorns. 
Off /a ndra FI (tribune! a. 
Plate XV., Pig. 2, shows a transverse section of a leaf of this 
plant. On the under surface of each leaf there are numerous pores 
leading into spacious llask-like cavities hollowed out in the spongy 
parenchyma. Stomata are restricted to the inner surfaces of these 
cavities. The remaining external surfaces are protected by a thick 
waterproof cuticle. As a result, the stomata are not exposed 
directly to the sun's rays and the air on to which they open is 
changed exceedingly slowly and soon becomes almost saturated 
with water vapour. Evaporation through the stomata, therefore, 
is greatly retarded. The efficiency of the system is still further 
enhanced by numerous hairs which arise from the epidermal cells 
lining each cavity and which produce a plugging effect at the pore- 
like opening. 
The mechanical tissues of the leaf are strongly developed, a 
layer of sclerenchymatous fibres underlying* the epidermis, in- 
creasing the distance between the softer tissues of the mesophyll 
and the leaf surface. This lessens the tendency for evaporation to 
occur, tempers the brightness of the sunlight and at the same time 
makes the leaves so hard that they feel like thin chips of wood. 
A slightly contradictory feature is the lens-like appearance of 
the epidermal cells overlying the groups of palisade cells. It 
would seem that they function in concentrating the light on the 
assimilation tissue beneath, hut probably they serve chiefly as water 
reservoirs close to the regions of greatest photosynthetic activity. 
A m m o j>h ila A rundinacene. 
The next xerophyte to be mentioned is Ammopkila arundinaeea, 
Host (the Marram (trass), examples of which may be found grow- 
ing close to the beach near Perth. Although not an indigenous 
plant, it is so well established as a naturalised alien that it is not 
out of place to mention it here. This grass T first gathered with 
other plants at the Osborne Rifle Range. On examining the long 
