3^3 



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 ana 

 desert plants may lie uprooted and exposed to the sun for 

 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 cas< ol succu- 

 lents, and, among other important functions is indisj as a 

 medium for the transportation and introduction into the plant, of 

 the nutrient substances occurring in the soil : the whole of the plant 

 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 dry 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 

 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 ofl the 

 ground is considerably higher: this however is counteracted to some 

 extent by the heavier annual rainfall. We thus see the value ot 

 \ shade trees" on newly cleared and planted ground, which by 

 breaking the force of the rain, and by lessening the amoui 



