68 



KNOWLEDGE 



[March 1, 1894. 



THE ROOT-TUBERCLES OF PEAS, BEANS, AND 

 VETCHES.-I. 



By J. Pentland Smith, M.A., B.Sc. 



A GREAT amount of interest has been excited of late 

 by the discovery of the function and use of the 

 nodules which are found on the roots of plants 

 belongmg to the natural order Ltinaninosce — 

 plants whose fruit is a pod or legume. 



The subject is interesting alike to the botanist, chemist, 

 and agriculturist ; to the botanist because of the biological 

 problems connected with it, to the chemist because of the 

 light it sheds on the complex chemical processes occurring 

 in the soil, and to the agriculturist from the practical 

 conclusions to be drawn from the data afforded by purely 

 scientific investigators. It is a fascinating subject from 

 whatever standpoint we choose to regard it. 



Agriculturists have noticed that the cultivation of one 

 species of plant on the same soil, year after year, is 

 detrimental to the well-being of plants ; and they have 

 found that if crops are grown in certain successions good 

 yields are the result. 



De CandoUe explained this by saying that the roots of 

 plants excrete into the soil a specific substance, injurious 

 to crops of the same kind. It became evident, however, 

 that by a system of rotation, not only were better crops 

 obtained, but a smaller quantity of manure was required 

 for their production. Liebig's explanation of the 

 benefits accruing from a rotation of crops seems more 

 acceptable than that of De t'andolle — in fact it is now 

 known, as the result of numerous analyses, to be the 

 correct explanation : one species of plant abstracts from 

 the soO larger amounts of certain salts than another. 



A short resume of the manner in which a plant furnished 

 with green leaves obtains its supply of food will form 

 a fitting introduction to this subject. The essential 

 materials of plant food are carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phofsphorus, potassium, sodium, &c. 

 This has been determined by an analysis of plant tissues 

 and of the Urinij part of the plant, the protoplasm. They 

 must either be obtained by means of the roots from the 

 soil in which the plant is growing, or taken from the air 

 by the leaves. Experiments have shown that both sources 

 are drawn on. The greater part of the carbon and oxygen 

 are obtained from the au- by the leaves, and the soil is the 

 • storehouse from which other elements are drawn. This is 

 rather startling when we recall the fact that, estimated 

 roughly, about four-fifths by volume of the atmosphere is 

 composed of nitrogen, about one-filth of oxygen, and the 

 rest chiefly of carbon dioxide (commonly called carbonic 

 acid gas). It has long been known that the green colouring 

 matter, chlorophyll, in the presence of bright sunlight, 

 possesses the power of breaking up carbon dioxide into its 

 constituent elements, carbon and oxygen ; the carbon and 

 part of the oxygen being retained by the plant, while the 

 remainder of the oxygen is given off into the atmosphere. 

 In performing this function of carbon assimilation the 

 plant is purifying the air. Though the counter process 

 of respiration which is going on continuously day and 

 night has the opposite effect, the oxygen evolved during 

 the day is in excess of the carbon dioxide produced dm-ing 

 that time by the respiratory process. 



An interesting experiment, de\-ised by Engelmann, 

 illustrates very forcibly the part played by chlorophyll- 

 bearing plants in the economy of the world. He projected 

 a fine spectrum of the sun's rays on a slide containing 

 a filamentous alga (that is. a thread-like sea- weed), and a 

 quantity of the bacterium, Bavtenum photometrkum , which 



has such a passion for oxygen that it collects in its hosts 

 at the edge of the cover glass, where there is a greater 

 supply of that element. The result was the assemblage 

 of masses of the bacterium in the red-yellow and blue- 

 violet parts of the spectrum, showing that carbon assimi- 

 lation was proceeding most actively at these points. 

 Green plants are thus, by virtue of their possession of 

 chlorophyll, the converters of the unorganized carbon 

 dioxide of the atmosphere mto a form available for plant 



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Fig. 1. — A filamentous Alg.!. on which the solar spectriuu' has 

 been projected. (After Engelmann and A'ines.) In llie water are 

 quantities of the bacterium, Sacffi'ium pIiofomefriciitH. w^hich have 

 aggregated tuem>*elves in the red-yellow and blue-violet parts of 

 the spectrum. This bacterium is an oxygen lover, and its accumu- 

 lation at these parts of the spectrum shows that carbon assimilation 

 is proceeding most actively in the regions where chlorophyll exhibits 

 the most marked absorption bands. 



food. And in the performance of this function they 

 convert the kinetic energy of the sun's rays into a potential 

 form, as Engelmann's experiment clearly shows. This 

 afterwards, in becoming kinetic, supplies the motive 

 power whereby the chlorophyllous plant is enabled to 

 carry on its life processes, just as the potential energy of a 

 wound-up watch spring, in changing into moving energy, 

 carries the works of the watch ; or the water, which 

 dammed up on the hillside is endowed with potential 

 energy in virtue of its position, assumes the kinetic form 

 when the sluice is withdrawn, and drives the mill-wheel. 

 This point is an extremely important one and must not be 

 lost sight of in the discussion which follows, for according 

 to this view of the case all non-chlorophyllous vegetable 

 organisms and all animals must directly or indirectly 

 prey upon chlorophyll-bearing plants, as they alone can 

 assimilate the carbon which enters so largely into the 

 composition of all living bodies. Hence all fungi and 

 other plants characterized by the non-possession of 

 chlorophyll must either be saprophytic (i.t., live on dead 

 organic matter) or parasitic {i.e., prey upon the living 

 bodies of other plants or animals). It is for this reason 

 the suggestion has been made that green-coloured plants 

 were the living forms first evolved from non-living matter, 

 although this view is negatived by Eay Lankester in his 

 article on " Protoplasm '' in the EncydoiuFdia Britannka. 



The water absorbed by the roots from the soil contains 

 small quantities of salts of potassium, calcium, kc, in 

 solution, and this is the source, as previously mentioned, 

 of the other constituents of protoplasm. These unite with 

 the assimilated carbon, and the resultant protoplasm is 

 the last link in the chain of complex organic compounds 

 formed by their union. The energy of the solar rays is 

 consumed in the production of plant protoplasm, and the 

 decomposition of this substance supplies the kinetic energy 

 that is the motive power of the life processes of all other 

 plants and of animals. 



During the last fifty years Sir John Lawes and 



