104 
(lias, the Westralian Christmas Tree , 8 the Sandalwood Tree , 9 the 
Quandong , 10 and Australian Mistletoes , 11 all make our bush a verit- 
able botanic garden of parasitic plants. 
This plentitude of parasites in our xerophvtic plant assemblages 
is worthy of note. It may he that the parasitic habit is as much (or 
perhaps more) due to the aridity of environment as to the stimulus 
of nitrogen search. The emphasis is still further removed from the 
nitrogen-seeking stimulus when it is remembered that in Westralian 
soils the essential soil constituent present in minimum amount is 
phosphorus — not nitrogen. Of course it may he shown that many of 
the parasites remove nitrogen containing compounds from their hosts, 
but this, though now of great importance to the parasite, may be 
quite a secondary phase of the habit. 
Again, in some instances, it may he noticed that a eassythn pints 
out hailstorm at places which are not in contact with any host. The 
parasitic attack must in these eases he an act innate in the parasite 
and not dependent on external stimulus. With evidence of such a 
change having been accomplished, it does not seem right to ascribe 
the origin of parasitic habits to nitrogen -seeking simply because a 
parasite may he shown to derive nitrogen from its host. 
Thus, although usually regarded only in its nitrogen relation- 
ship, plant parasitism in Western Australia possibly originated as 
a modification capable of adjusting some disparity between water 
absorption and water loss. The securing of nitrogen would he at 
first a non-essential concomitant of successful attempts to secure 
water from better provided neighbours, and the highly specialised 
parasites exhibit the ultimate developments of a trend of evolution 
which originated in some stimulus of water relationship). 
References to plates. 
P. stomatal cavity. 
O. T. curvature tissue. 
Cu. - - cnticie. 
K. epidermis. 
(10. guard cells. 
H. hairs. 
Ij. lens-like epidermal cells. 
M. mesophyll cells. 
M.P. palisade mesophyll. 
M.S. spongy mesophyll. 
Sc. sclerencliyma. 
S.P. secondary furrows. 
S.I\ stomatal pit. 
V. B. vascular bundle. 
W. P. water-storing parenchyma. 
X. - entrance to furrow. 
Y. air above leaf becoming enclosed by rolling. 
P. fibres underlying epidermis. ( In Plate XVIT. layer is almost 
continuous below stomata.) 
Pi fibres surrounding water-storage tissue. 
8, Jfnytsiii Floribinida 
9. Pusanus Spi.-nl us. 
XI. Tiorant'bacca;. 
10. Fnsamis Acuminates, 
