352 THE LIFE OF THE PLANT 



this rule, or transforms into useful work all the energy obtained ; 

 and this consideration alone is sufficient to prove that the 

 physiological limit of vegetable productiveness cannot coincide 

 with the physical. Contrary to the figures just mentioned, 

 which have been taken from the results of various cultures, an 

 objection of the following kind can be brought forward : although 

 field vegetation, as we have seen, presents a highly developed 

 surface for absorption, we cannot, however, say that it absorbs 

 all the sunlight that falls upon it. The following experiment 

 will give us from this point of view more trustworthy statistics. 

 By exposing to sunlight green leaves with a surface of accurately 

 measured area, determining by means of analysis the amount 

 of carbonic acid decomposed by this leaf in the best light, say 

 during the space of an hour, and determining also the amount 

 of heat that falls upon the selected surface of the leaf during 

 that hour, we shall obtain all the data necessary for calculating 

 the correlation between the absorption of energy and its utilisa- 

 tion in decomposing carbonic acid. Direct experiments of this 

 kind gave on the average ^hs^^ of ^ the energy received, 

 /jth at the best. Some recent calculations show that this 

 quantity can be increased to ^th. This last figure may probably 

 be considered as approaching the limit of physiological pro- 

 ductiveness, because the plants in these experiments were 

 placed under the most favourable conditions possible. Thus 

 we see how closely aU our most intensive cultures approach what 

 we have called the physiological limit, i.e. the largest amount 

 of organic matter which can be obtained by means of a plant 

 from a given area of land. 



Even at this limit, however, only y^th and in the best instance 

 ^\th of the energy received is retained. This wiU cease to puzzle 

 us if we consider the fact that apart from this uniquely pro- 

 ductive work from man's point of view, other work takes place 

 in the plant, entirely unproductive for man.i In the first place 

 the plant evaporates during the whole of its life-time such great 



' It is still more important to take into account the fact that a leaf can- 

 not even absorb all the light of the sun : otherwise it would be black 

 instead of green. Modem investigations prove that the leaf absorbs on 

 the average 25 per cent, of all the radiant energy received from the sun 

 — this is the physical limit; 3-3 per cent, are utiHsed in physiological 

 experiments, and i per cent, in the fields. 



