49 
organs, are highly specialised to cope successfully with drought, 
while the great majority of our species show some indication in 
their structure of the necessity that exists for special provision 
against this feature of the climate. In considering the question 
of how this is effected it is necessary to study the relation of the 
plant to water, the element of most vital importance to it. Plants 
live and grow by the absorption of water from the ground by 
means of their roots, which serve also to keep them in a fixed 
position. This water finds its way through the stem and branches 
to the leaves, from the surface of which it is exhaled into the 
air. It is not always pure when it enters the plant, but contains, 
dissolved in it in small quantities, a number of different mineral 
substances necessary in the formation of its tissues. Sometimes, 
however, these ingredients in the water are prejudicial to the 
growth of the plant, and may cause its death ; in other cases 
they may be tolerated or even favorable to growth, while in others 
again they may be the cause of peculiarities of structure sufficiently 
pronounced to have caused botanists to describe as distinct 
species, plants so modified by the absorption of water charged with 
a particular mineral substance derived from the soil in which 
they grew. These, substances in the soil may be injurious to the 
majority of other plants, so that toleration of them by a particular 
variety may be the means of its advancement and spread, through 
being relieved from the competition of other plants in that soil. 
The great bulk of the woody tissues of the plants, however, 
is not derived from the water supplied through the roots, but 
from the atmosphere, which contains carbonic acid gas, the 
source of the element carbon, the chief constituent of wood. 
The extraction of carbon is effected by the vital action of the 
leaves in their performance of the function of respiration. In a 
large tree bearing many thousands of leaves it can be understood 
that a very great quantity of water is required for its growth, 
but if the soil does not contain much or soon becomes exhausted 
the growth will be proportionately restricted. In soils that are 
very dry or liable to become so at particular seasons plants must 
be modified in some such way as will enable them to do with a 
scanty supply of water. One obvious way in which moisture 
may be secured, even during the dry season, is by an extension 
of the root system to the deeper strata of the soil, which are 
less subject to the desiccating influence of the sun and atmosphere 
than the superficial layers. As we have a very long dry summer 
we should expect to find the great majority of our native perennial 
plants to have roots long enough to reach deep into the ground. 
Examples of this may be found by anyone who will take the 
trouble to dig deep enough. As an example, let me mention 
Calythrix flavescens, a small shrub less than a foot high, that 
makes the sandy scrub gay with its bright yellow flowers during 
the hottest months of summer, when the majority of flowers 
