THE LIFE -HISTORY OF PLANTS. 



217 



On the moors of Scotland, Angus Smith found 

 20*99 per cent, of oxygen in the atmosphere, as 

 contrasted with 20-74 in the pit of a theatre, or the 

 tunnels of the Metropolitan Railway, and 20-14 in 

 mines. When the volume falls to 18-50 the light of 

 candles is extinguished. Carbonic acid varies in 

 proportionate volume, according to the same observer, 

 from 2-50 per cent, in some Cornish mines to as little 

 as -033. The significance of these figures will 

 become more apparent further on ; all that need be 

 said here is that by the agency of the respiratory 

 process the plant obtains the oxygen necessary to 

 insure the activity of the protoplasm. Without it 

 life is brought to a standstill by suffocation, just 

 as in the case of an animal similarly deprived. 

 Every separate portion of the plant, moreover, must 

 have its own independent supply, and cannot trust 

 to receive it from any other portion. There are, 

 indeed, certain Bacteria which appear to thrive with- 

 out oxygen ; but we need not further allude to these 

 exceptional cases, as they do not, so far as we at 

 present know, concern the ordinary gardener. There 

 is at the present time a difference of opinion between 

 chemists as to whether the process of respiration in 

 vegetables is identical with that in animals, or only 

 correspondent, for while some assert that there is no 

 fixed relation in plants, as there is in animals, be- 

 tween the quantity of oxygen absorbed and the 

 volume of carbonic acid emitted, others, like MM. 

 Bonnier and Mangin, maintain the reverse. The 

 production of the carbonic acid gas from leaves when 

 they are plunged in an atmosphere of hydrogen or 

 nitrogen without any oxygen at all, is a proof that in 

 these cases the elimination of the carbonic acid is 

 not a respiratory act, but the result of changes in the 

 composition of the tissues of the plant. 



The proportion of oxygen absorbed increases 

 with the temperature, so that it may be said, 

 if we required to make a plant breathe deeply, all 

 we have to do is increase the temj)erature. But 

 the amount of oxygen absorbed varies under like 

 conditions in different plants. Succulent plants like 

 Agaves absorb least, evergreen leaves more, and 

 ordinary deciduous leaves most (De Saussure). A 

 flower takes up more oxygen in proportion than 

 a root or a leaf, and the stamens of a flower take up 

 more than the outer parts. Young leaves of the 

 same plant absorb more than older ones. The effect 

 of this continual influx of oxygen is to " oxidise " 

 certain of the substances in the plant, and to give 

 i-ise to changes of the utmost importance, but which 

 it falls to the lot of the chemist rather than of the 

 botanist to explain. 



Two points, however, we must allude to as ne- 

 cessary concomitants or consequences of oxidation, 

 and these are tlie elimination of carbonic acid gas 



concurrently with the inhalation of oxygen, and 

 the production of heat. The entrance of oxygen 

 and the emission of carbonic acid gas are simply the 

 beginning and the end respectively of a long and com- 

 plicated series of changes in the plant —changes but 

 very imperfectly known at present. These chemical 

 transmutations are attended with the production of 

 heat. A certain amount of heat is requisite to 

 insure the activity of the molecules or ultimate 

 particles of the protoplasm, and if this De not 

 forthcoming, the protoplasm remains inactive, and 

 sooner or later dies. This internal heat generated 

 in the living plant (or animal), and essential to its 

 life, is a necessary consequence of oxidation. Oxida- 

 tion, then, maintains the internal heat of plants, and 

 while it is the source and origin of some substances 

 within the plant, it tends at the same time to de- 

 stroy the tissues in the same way that they would 

 be destroyed by fire, but of course without flame. 

 Respiration is thus a destructive process, which of 

 itself would destroy the plant were the waste so 

 caused not compensated for by the nutritive pro- 

 cesses. 



Chlorophyll. — Perspiration, as has been stated, 

 takes place, subject to a few exceptions, from 

 all or any part of the surface of a plant, by day 

 and by night ; in plants like f imgi, which have 

 no green matter, as well as in those which do 

 possess it. In those plants, however, which contain 

 green matter (chlorophyll) changes go on during the 

 day-time (that is, when the green matter is exposed 

 to the light) of exactly the opposite nature to 

 those mentioned under the head of respiration. 

 Oxygen gas is now given off and carbon is now 

 retained in the plant to build up its structure. 

 If a leaf be placed in water and exposed to the sun, 

 in a very short time bubbles of gas may be observed 

 to collect on the leaves, and by appropriate chemical 

 means it may easily be shown that these bubbles 

 consist principally of oxygen gas. Here again the 

 aid of the chemist is necessary, and for the full con- 

 sideration of the matter recom^se must be had to 

 chemical works. 



Food of Plants. — The substance of which the 

 tissue of plants is composed consists of " cellulose," 

 and is composed of certain proportions of carbon, of 

 oxygen, and hydrogen, in combination with water. 

 This cellulose forms the membrane of the cells in 

 the cell-walls, within which is the protoplasm before 

 alluded to, and which latter contains, in addition 

 to the substances just mentioned, a quantity of 

 nitrogen. Besides these fom- elements just men- 

 tioned, which are indispensable to all plants, as the 

 constituents of cellulose and protoplasm, there 

 exist minute quantities of phosphorus and sulphur, 



