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those of a typical acacia. Those formed next show some reduction in 
the surface area of the small leaflets, while a compensating expansion 
o I the leaf stalk enables photosynthetic activity to he maintained. 
Lastly, foliar growths consist of expanded leaf-stalks only, no trace 
of the acacia-like leaflets being displayed. We here see each individual 
climbing its genealogical tree, showing clearly a progressive reduc- 
tion of leaf surface, which must have accompanied the growing 
aridity of environment. 
It seems, then, that external factors are able to affect the body 
tissues. These, in their turn, must influence the metabolism of germ 
plasm and so cause variation from generation to generation. 
A Darwinian progressive variation such as that suggested above 
does not exhaust possible explanation of the origin of xerophytic 
structures. Change of external conditions, or wide interspecific 
crossings, may cause changes in number, function or chemical com- 
position of chromosomes or more general changes in the protoplasm 
of a cell or cell nucleus. The progeny of plants so disturbed exhibits 
abrupt and distinct character changes, not wholly explainable by 
any regrouping of old characters. The evening primrose, Oenothera 
la march ktna, carried from the New World to the Old, was thrown 
by the change into some state of nuclear instability. Three varieties 
appeared, each breeding true. Variety gigas has twenty-eight, chro- 
mosomes instead of the normal fourteen, and has an accompanying 
structural characteristic, a greatly increased size. Variety semigigas 
exhibits the triploid number of chromosomes, whilst variety lata 
has fifteen. Environmental change has been accompanied by nuclear 
disturbance, an outward sign of which is the changed structure of 
the individual. 
Luther Burbank, experimenting in specific plant crosses, pro- 
duced entirely new varieties of plants. The union of species of 
dewberries and raspberries resulted in the primus and phenomenal 
berries — fruits with new size, colour, texture, and taste characters. 
These proved to breed true. The plumeot, a cross between plums and 
an apricot, does not appear to have become fixed. 
In nature, combinations of circumstances are known to produce 
mutants. Darwin, calling them “sports, 5 * attached too little import- 
ance to them in his view of biological evolution, while De Vries, 
passing to the other extreme, held that evolution had progressed 
solely through their instrumentality. Both views have proved nar- 
row. Xerophytes have developed in each wav, and, should a com- 
plete suite of Western Australian fossils ever be available, some 
plants will be traceable, step by step, back to their hygrophilous 
ancestors, whilst others will prove to have assumed drought-resistant 
characters in an abrupt manner. 
One further phase of xerophytism remains to he mentioned, and 
that is its relationship with plant parasitism. A profusion of cassy- 
