PHYSIOLOGICAL DROUGHT. 
3 
latter making for success in our epoch—witness the relative sub¬ 
ordination of the gymnospermous to the angiospermous type. 
Shoot-form and Water-conservation. —Our grouping of plants 
as hygrophytes, tropophytes, xerophytes, and recognition of chylo- 
phylly, chylocauly, ericoid habit and the like, bear witness to our 
appreciation of the influence of the water-relation upon shoot- 
form, internal and external, and tell of its adaptation to water- 
conservation.' We know indeed now so much of the meaning of 
shoot-form in respect of this factor that we can predicate from 
its aspect whether a plant is the inhabitant in nature of a physio¬ 
logically dry or physiologically moist station, and obtain thereby 
a definite index pointing to the nature of the treatment that should 
be extended to it in cultivation. We have in fact advanced in our 
interpretation of adaptations of the shoot to water-supply and 
now appreciate indications finer than those offered (and acted 
upon by the gardener) in the distinction between aquatic plants 
on the one hand and terrestrial and epiphytic plants on the other. 
Root-form and Water-obtention. —Root-form in relation to ob- 
tention of water has received perhaps less attention—except in 
the case of the epigeous and more visible roots of epiphytes— 
and as a diagnostic mark of natural physiological habitat is, as 
a whole, at least with our present knowledge, not of such con¬ 
spicuous value in affording a guide to horticultural practice as 
shoot-form. Moreover many of its distinctive features in rela¬ 
tion to water being microscopic they can never be used in practice 
by gardeners as a test of station. Yet a proper understanding 
of the ecology of soil-roots as well as of air-roots will give the 
gardener an explanation of phenomena that puzzle him. 
Absorbing and Transmitting Mechanism. —Within the limit of 
general form imposed by their surroundings, roots are variable 
and adaptive as are shoots and show remarkable correlations of 
extent of absorbing area with absorbing and transmitting mechan¬ 
ism in relation to soil-environment and shoot-needs. The story 
of these is a chapter of root-ecology which has not yet found a 
writer, so far as I know; when written it will bring out no less 
distinctive adaptations than in the case of the shoot. The paper, 
the lines of which I now signalize, contains a contribution to that 
