3 % 
WATER AND ITS RELATIONS TO 
PLANT LIFE. 
Of the various factors affecting plant life, none is more important 
than water. Without water there can be no life, though the 
amount necessary to bring about full development varies very 
considerably ’in different plants. As the conditions under which 
plants live are not constant, we find a corresponding variation in the 
manner in which they have adapted themselves to the varying con- 
ditions ; thus while some plants such as pine-apples, cacti and othe - 
drsert plants may lie uprooted and exposed to the. sun for weeks 
without suffering any injury, others such as aquatics are quickly 
killed by exposure to a moderately dry atmosphere. 
Water forms the chief constituent of living plants, amounting to 
no less than 96 per cent, of the total weight in the case of succu- 
lents, and, among other important functions is indispensable as a 
medium for the transportation and introduction into the plant, of 
the nutrient substances occurring in the soil : the whole of the pknt 
food obtained from the soil entering the plant through this medium, 
while its elements — hydrogen and oxygen — also enter into the 
various organic compounds of plant life. 
A large amount of the water taken up by the roots from the soil 
is retained by the plant; though a still larger quantity is transpired 
through the leaves, the various salts and other substances held in 
suspension being deposited in the plant. The actual quantity of 
water evaporated by the leaves is enormous ; it has been calculated 
for example, that a well developed Birch tree standing perfectly 
free, would lose by evaporation on a hot dry day over 400 litres of 
water. An ordinary field crop transpires about 300 lbs. of water for 
each pound of drv matter produced ; so that in the case of an acre of 
marigolds yielding say 30 tons at harvest, and containing 88 per 
cent, of water, the amount of water transpired during growth would 
be no less than 1,080 tons per acre. The amount of. water trans- 
pired by hops during growth, reaches from 3-4,000.000 litres per acre. 
In the British Isles, this represents more than half the total an- 
nual rainfall, so that when we consider that a large percentage of 
the rain runs straight off the ground and is lost to the plant, and 
that a further quantity is lost by evaporation from the soil, we shall 
see how, even in a humid climate, the available water supply may 
easily fall below the amount necessary to bring about maximum 
development. This is equally true of the tropics, where the loss due 
to evaporation is much greater than in temperate regions, and 
where, owing to the amount of rainfall in a given time being rela- 
tively greater than in countries beyond the equatorial zone, the loss 
of water to the plant through the water running straight off the 
ground is considerably higher : this however is counteracted to some 
extent by the heavier annual rainfall. We thus see the value of 
,f shade trees’-' on newly cleared and planted ground, which by 
breaking the force of the rain, and by lessening the amount of 
