SCIENCE-GOSSIP. 



63 



this view. In the first place it is the heat from 

 the furnace of the engine which is the ultimate 

 source of its work. The same is true of all living 

 things, whether plants or animals, or human 

 beings. That we are warmer than our surround- 

 ings is an evident fact, and this warmth is 

 generated from what we may truly describe as a 

 smouldering fire spread over the whole length and 

 breadth of our bodies. In this burning of our 

 body, e.xactly the same gases are produced as in 

 the burning of a candle or of a fire. It is this 

 process of slow combustion which allows us to 

 move, and to think, and to act ; indeed, which gives 

 us our lives. It drives us, just as the fire of the 

 furnace drives the engine. 



Precisely the same holds true for plants. It is 

 easy to show that they are warmer than their 

 surroundings, and that they are constantly genera- 

 ting the same gases as are to be found in our 

 breath or in the vapours surrounding an ordinary 

 fire. When a candle burns, its substance wastes 

 away and is gradually disseminated in the atmos- 

 phere in the form of gases. This loss of substance 

 is the invariable accompaniment of combustion, 

 and is to be found in our own living bodies or in 

 those of plants as much and as plainly as in a 

 burning coal or in a lighted taper. The great 

 distinction between the process of combustion in 

 an inanimate substance and that going on in a 

 living body — breathing as we there term it — lies in 

 the fact that the former burns itself, often fiercely 

 and quickly, entirely to invisible gases, whilst the 

 latter, viz., the living body, has every particle 

 it thus loses recompensated by a balancing 

 process which is always going on side by side with 

 respiration. We, all of us, are familiar with the 

 sensation of hunger. This is Nature's means of 

 telling us that we have lost as much of our bodily 

 material as is good for us, and that we must now 

 by some means restore what we have lost. I need 

 not add that this restoration is effected by the 

 process of feeding. In plants, likewise, the 

 phenomenon of feeding or nutrition is the agent 

 balancing the loss of substance undergone in 

 breathing. 



It is to this characteristic and important pheno- 

 menon of living things, as we find it in the vegetable 

 kingdom, that we must now turn. Since we have 

 seen that nutrition is the factor restoring the plant 

 body to its original weight after this has decreased 

 through the necessary processes of respiration, it is 

 evident that in seeking the nature of the food of 

 plants we have an excellent guide in the nature of 

 the plant substance itself. Examination will show 

 us that the chief and most important constituent of 

 the plant body is a compound of three simple 

 chemical elements : carbon, oxygen and hydrogen. 

 Carbon is familiar to everyone as charcoal, oxygen 

 as one of the gases of the air, and hydrogen as a 



D 



somewhat similar gas which can be prepared from 

 water. If a plant can obtain such a carbon - 

 compound as this, it can readily join it to other 

 things and build it up into its substance. 



Looking at the surroundings of the plant, we see 

 that the soil into which its roots dip is, or should 

 be, soaked with water up to a certain limit. The 

 air which envelops the shoot also is found to 

 contain a certain proportion of carbonic acid gas. 

 If either of these two things be entirely withheld 

 the plant flags and then dies. Analysing water 

 and carbonic acid gas chemically, we see that the 

 former consists of hydrogen and oxygen, the latter 

 of carbon and oxygen ; in other words, that between 

 them they contain the same three elements as the 

 carbon-compound above mentioned as forming food 

 stuff. Further study shows us that the water of 

 the soil enters the plant through the hairs which 

 clothe the root, that it then passes into a system of 

 pipes or vessels which convey it up the stem to the 

 leaf, and that in the tissues of this organ it meets 

 the carbonic acid gas which has found its way 

 from the air through the pores which cover the 

 surfaces of the leaf. Examined microscopically, 

 the leaf shows itself to be filled with numberless 

 green granules, called chlorophyll grains ; these 

 have the very highest importance in vegetable 

 nutrition, and are the cause of the green colour 

 which foliage leaves nearly always ha\e. As the 

 sunshine falls upon them, these chlorophyll grains 

 have the power of absorbing it ; they act as so many 

 little traps to the light, and just as sunlight falling 

 upon a photographic plate effects wonderful changes 

 in it, resulting in the production of a picture, so 

 does this same light when caught and directed by 

 the chlorophyll grains of the leaf, also bring about 

 extraordinary alterations in the materials lying 

 around and bathing the granules. These substances 

 are water and carbonic acid gas, and the effect of 

 this chlorophyll directed light is to bring about the 

 chemical union of these two things to form such a 

 carbon-compound, as I have already mentioned, as 

 a useful food material for the plant. This com- 

 pound, which first visibly makes its appearance in 

 the substance of the green grains, is called starch ; 

 it is not quite the first substance to be formed, but 

 it is the first which we can plainly see. It is 

 afterwards dissolved and united with yet other 

 things that have been absorbed from the soil, such 

 as nitrogen and sulphur, and finally built up into 

 the living material of the plant itself, compensating 

 it for its loss through respiration. 



I mentioned earlier in my paper that a living 

 plant or animal in many respects resembled an 

 engine ; but we now come to a striking point of 

 difference between the two things. The engine 

 is rigid and inflexible, whether the air around it be 

 warm or cold, whether in darkness or light, 

 whether in bright sunshine or in rain, the same 



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