APPENDIX 353 



■quantities of water that in amazement we are inclined to dis- 

 credit the figures. The evaporation of this quantity of water 

 apparently requires much more heat than is used in the decom- 

 position of carbonic acid. Therefore, together with the pro- 

 ductive work in the formation of organic matter, the plant 

 uses still more energy in work useless to man — in evaporation. 

 But this, although the most important, is not the only other 

 expenditure of energy in a plant. The plant absorbs that water 

 from the soil, and therefore has to raise it to a certain height. 

 This work may be expressed in foot-kilogrammes. It may be 

 neglected in the case of our field-plants, but is considerable 

 in our trees.^ We can imagine what a large amount of 

 work underlies the raising of the masses of water evaporated 

 in forests by giant trees like the eucalyptus of Australia, 

 the tops of which, according to one botanist, might have cast 

 their shadow even on the summit of the pyramid of Cheops. 

 However, evaporation and the raising of water are not 

 achieved solely by the energy derived immediately from the 

 heat of the sun, though a considerable amount is certainly 

 supplied in this way. To these causes of unprofitable waste of 

 the sun's energy must also be added another. We cannot 

 avail ourselves of all the organic matter stored up by the plant 

 during its life-time, because it uses, burns down, part of that 

 matter itself. We can say that it uses in this way as much as 

 ^th of aU the matter, so that as regards the accumulation of 

 organic matter the plant makes twenty steps forward and one 

 backward. 



AU these causes of waste of the sun's energy which we 

 have enumerated illustrate for us, so to speak, the expenses 

 of the production of organic matter by the plant. We See, 

 therefore, that although the plant is a very perfect apparatus 

 for utilising the energy of the sun, it nevertheless leaves much 

 to be desired, since at the best it transforms into work useful for 

 man only y^th or a^ifth of all the energy it obtains from the sun. 

 Man has to face the problem either of perfecting the plant in this 

 respect, or of inventing in its place an artificial apparatus, which 

 shall utiHse a greater percentage of the energy acquired, and 



1 The raising of the sap to a great height may be considered as unpro- 

 ductive, only with regard to the production of matter, but on the other 

 hand it is all-important as it furnishes us with timber, 



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