198 



KNOWLEDGE. 



[AfGusT 2, 1897. 



Carbonic acid is a compound of carbon and oxygen : 

 under the influence of sunlight chlorophyll has the pro- 

 perty of absorbing and decomposing this gas ; the combina- 

 tion is broken up, the oxygen is returned to the air, and 

 the carbon, retained by the chlorophyll, is employed in the 

 building up of vegetable tissue. Carbon separated from 

 the air in this way constitutes the most important part of 

 the food of plants. In the dark, plants are incapable of 

 extracting carbon from the air ; they are dependent on 

 light, which acts as a reducing agent on the carbonic acid 

 absorbed by the leaves. 



After it has been blown through among leaves, air is 

 found on analysis to contain less carbonic acid and more 

 oxygen than it did before. A few minutes' contact with 

 the leaves of the vine is sufficient to deprive air entirely 

 of its carbonic acid. If a bunch of mint be placed under 

 water on which the sunlight is playing, bubbles of gas are 

 seen to escape from the leaves, which, on examination, 

 proves to be pure oxygen. Such facts sufficiently explain 

 Dr. Priestley's results ; the leaves of the plant absorbed 

 the carbonic acid as fast as it was exhaled by the mouse, 

 and substituted an equal volume of free oxygen, so that 

 the composition of the air in the jar remained throughout 

 the same as it was at first. 



Animals, it thus appears, are indebted to vegetables, not 

 merely for food, but likewise to a great extent for the air 

 they breathe. " The oxygen we are breathing," wrote the 

 late Prof. George Wilson, " was distilled for us some time 

 ago by the magnolias of the Susquehanna and the great 

 trees that skirt the Orinoco and the Amazon. The giant 

 rhododendrons of the Himalayas contributed to it, the 

 roses and myrtles of Cashmere, the cinnamon trees of 

 Ceylon, and forests older than the flood buried deep in the 

 heart of Africa, far behind the Mountains of the Moon. " 

 The relationship is, however, one of mutual dependence 

 and benefit, for animals feed plants with carbon, and that 

 in a gaseous combination suited to their powers of assimi- 

 lation. The mouse in Dr. Priestley's experiment supplied 

 the plant with both carbon and oxygen, but only received 

 back the oxygen ; this unequal exchange in the long run 

 would necessarily mean the gradual transference of all the 

 animal's substance to the tissues of the plant. This is, 

 indeed, the ultimate destiny of all animal substance, for 

 not only are vegetables the natural heirs of effete and 

 refuse animal matter, but the carbon atoms of which any 

 plant is built up, we may safely say, have all at one time 

 or other formed part of some animal. Since plants are 

 constantly receiving carbon from animals, the latter must 

 refund their losses if their existence is to continue. To 

 counterbalance its loss we must imagine the mouse in the 

 experiment to devour a portion of the plant containing as 

 much carbon as was eliminated in respiration. Even when 

 accounts are thus squared, animals are still indebted to 

 plants for the energy generated by the combustion of their 

 food. 



In the body of an animal there is always going on a 

 process of slow combustion ; oxygen taken in by the lungs 

 enters into union with carbon derived from the food, 

 carbonic acid is formed, and heat evolved. Every atom of 

 carbon disengaged from oxygen by the plant represents an 

 expenditure of heat upon the leaves equal to that developed 

 during its combination with oxygen in the animal system. 

 In the leaves this heat disappears ; it becomes latent, and 

 is stored up as potential energy. When we burn a piece 

 of wood, not only is the carbon which the tree originally 

 extracted from the air restored to the atmosphere in the 

 form of carbonic acid, but the heat given out by the 

 burning wood corresponds exactly to the amount by which 

 the sunbeams were chilled through the activity of the 



leaves while the wood was growing. Bobbing the sunshine 

 of its heat in this way, leaves must act as refrigerators ; 

 leaves exert a farther cooling influence on account of the 

 watery vapour they are constantly giving ofJ, for whenever 

 water evaporates much heat is rendered latent, and ceases 

 to be discerned by the senses. 



A man is said to exhale about one hundred gallons of 

 carbonic acid in a day. Boussingault estimated that a 

 square yard of leaf surface, reckoning both the upper and 

 lower sides of the leaves, can under favourable circum- 

 stances decompose rather more than a gallon of carbonic 

 acid in a day. At this rate, one hundred square yards 

 of leaf surface should keep a man going with oxygen. A 

 single tree would suffice for quite a number of people, 

 for according to the late Prof. Asa Gray's computation, 

 even a moderately sized elm possesses five acres of leaf 

 surface. 



Widely as they differ in appearance, a lung and a leaf 

 are constructed on very similar principles. In the lungs 

 the air tubes are subdivided into an infinite number of 

 small ramifications, around which the blood-vessels are 

 distributed in an extremely fine network. By this arrange- 

 ment an immense surface is exposed to the air, with 

 comparatively small expenditure of space. Through the 

 delicate membranous walls of these fine tubes the air 

 penetrates into the blood-vessels beneath, and from the 

 blood carbonic acid diffuses into the air tubes, and is 

 exhaled. The typical leaf, again, is a thin expanded plate, 

 offering a large surface to the air relatively to its mass ; 

 its lower stratum is honeycombed with innumerable 

 air spaces and intercellular passages, which communicate 

 with the external air through thousands of pores or 

 stomata. Oxygen and carbonic acid are exchanged 

 through the walls of the leaf-cells very much as in the 

 lungs, but in the reverse order. The flattened form of the 

 leaf secures illumination and promotes evaporation ; lungs, 

 having no need of light, are shaped to suit the space 

 available. The resemblance between the submerged 

 leaves of aquatic plants, such as the water-buttercup, and 

 the external gills or branchiif of the young tadpole 

 and other amphibians, is particularly striking. Both 

 organs are cut up into fine, thread-like segments, giving 

 them a fringed or feather-like appearance ; the large 

 absorbing area thus presented to the water facilitates the 

 absorption of the dissolved gases. Leaves are, however, 

 organs of nutrition. This resemblance to respiratory 

 organs arises entirely from the fact that by far the largest 

 proportion of a plant's food is absorbed in the gaseous 

 condition. Eeferring to this adaptation of the leaf, 

 Johnstone, in his " Agricultural Chemistry," says : " How 

 beautiful is the contrivance of the expanded leaf! The 

 air contains only one gaUon of carbonic acid in two 

 thousand five hundred, and this proportion has been 

 adjusted to the health and comfort of animals, to whom this 

 gas is hurtful. But to catch this minute quantity, the 

 tree hangs out thousands of square feet of leaf in perpetual 

 motion, through an ever-moving air ; and thus by the con- 

 joined labours of millions of pores, the substance of whole 

 forests of solid wood is slowly extracted from the fleeting 

 winds." 



Chlorophyll itself owes its formation to light ; but to 

 conceive how the ethereal undulations, acting on the living 

 substance of a leaf-cell, can elaborate a structure so 

 complex, baffles imagination. How marvellous are the 

 properties of Ught and how manifold its adjustments to 

 differently constituted materials ! Light is at once the 

 most useful of natural agencies and the most beautiful of 

 physical phenomena. And yet it must be tempered to 

 our vision. Painful and blinding the glare from rock or 



