4 8 M. C. St opes. 
Cretaceous descendants, is a position that seems utterly untenable* 
For an explanation of their structure let us turn to the living 
plants themselves. We see numbers of Gymnosperms choosing to 
live among the deciduous forest trees, though they are apparently 
qualified to inhabit more arid regions. The explanation of 
inherited xerophily does not seem to bear looking into closely. Are 
there any other factors apart from environment which might 
necessitate xerophily in a group of plants? 
The great influence of the surroundings and climatic conditions 
on plant structure is so universally recognised that we are liable to 
forget the still stronger influence on its form of the specific 
possibilities of the plant itself. 
If, as I now suggest, the Gymnospermic “ xerophily ” is not the 
result of even inherited adaptations to dry conditions, is not in fact 
an Ecological adaptation in the usual sense, but is a result of their 
histological structure (which is incapable of allowing a rapid flow of 
water through the wood), then the plants must set strict limits to 
their leaf surface and transpiration. Hence, even though they are 
growing with leafy deciduous trees in a mesophytic community 
well supplied with water, they are in individual want of a sufficient 
flow of water to allow of anything but what we call a typical 
“xerophytic” foliage. 
We must, it appears, distinguish between a xerophily which is 
the result of environment or of a past environment, and that 
which is the result of the individual limitations of the plant. 
Let us turn to some phylogenetic and physiological facts 
which bear on the special case of the Gymnosperms. 
From the study of Fossils we know that the Gymnosperms 
are a very ancient group, immensely older than the flowering 
plants. We also know that they are a more primitive group, and 
come in systematic position between the ferns and the flowering 
plants (properly above the Pteridosperms). Histologically we 
know that they have characteristic wood, entirely consisting of 
tracheides, which are usually pierced by “bordered pits”; the 
diameter of the tracheides is less than that of the vessels of the 
flowering plants, and the whole structure of the wood 1 is simpler 
and more uniform. Gymnosperms in fact represent plants in 
which the woody conducting system had not reached the state of 
specialisation and efficiency which was afterwards attained by 
Angiosperms. 
The work of plant physiologists has shown the relative 
' “Wood” is taken to cover the parenchyma, medullary rays, 
tracheides, vessels, and all accessory tissues of the xylem. 
