Mat 1, 1900.] 



KNOWLEDGE. 



loi 



an experieuce, iu its ■way, as that of crossing the other 

 " Line," to which I have alluded above. 



But are not these things written of in " Tramps 

 Abroad," and other such litoraturo? They need not, there- 

 fore, be further dwelt on here, as the matl-cr to which I 

 wish pai-ticularly to draw attention is the actual position 

 of the Date Line in its course from the Arctic to the 

 Antarctic regions. This is necessarily aiTccted by the 

 configuration of the continents, and by the groups of 

 islands contiguous to the ISOth meridian. But it is 

 obviously most convenient that the Date Line should 

 follow that meridian as closely as political and geo- 

 graphical circumstances will admit. The positions of 

 the Line, as assigned by the different authorities I 

 have been able to consult, are laid down on the accom- 

 panying map, which is reproduced, by permission, from 

 the '' Journal of the British Astronomical Association, ' 

 Vol. X., No. 4. The first thing that strikes one on 

 looking at the map is the divergence of the different 

 authorities. Notably the position given in Stieler's 

 Hand Atlas is discordant, and, at the same time, de- 

 viates most from the 180th meridian. The Atlas is, 

 however, dated 1892, and this position may perhaps be 

 considered, if not obsolete, at least obsolescent. The line 

 mai'ked " Wharton " is due to Admiral Sir W. Wharton, 

 the Hydrographer of the Navy; that mai'ked " Smith " 

 is taken from an ai'ticle in the '' Century Magazine " 

 for September, 1899, by Mr. Benjamin E. Smith; that 

 marked " Davidson " is due to Professor Davidson, of 

 the University of California. It will be remaa-ked that 

 Wharton and Davidson agree veiT closely, except in one 

 or two unimportant details, affecting a small group of 

 islands. By adopting either of these lines, it may be 

 assiuned, with some confidence, that we know " Where 

 the Day Changes,' except, indeed, for the gioup of 

 islands referred to, for which we must, I fear, for the 

 present remain in doubt. 



The further assimilation of the Date Line to the 

 180th meridian, though desirable, is diflicult of realisa- 

 tion, as the position of the Line depends on the con- 

 figuration of the pai'ts of Asia and America concerned, 

 and on the various circumstances which deteraiine the 

 direction in which the different groups of islands have 

 intercourse with the outer world. But, judging from 

 the past, progress in that direction, though slow, is sure, 

 and will eventually give us a better approximation to 

 a " straight " Date Line than we have at present. 



Would it be to inquire too curiously to ask where the 

 twentieth century begins? As to when it begins, we 

 have recently, somewhat to my astonishment, had a 

 Battle of the Centuries, in which one at least of the 

 crowned heads of Europe has taken a side, and has taJcen 

 the wrong side too I 



The answer to the question, Where does the day 

 change? also answers the question. Where does the 

 century begin? And, as we have seen, a fairly definite 

 reply can be given by a reference to the map, except 

 in the case of a certain group of islands. When does 

 the century change for that particular group ? Ah, well ! 

 it would puzzle even a crowned head to answer that 

 question. 



♦ 



PLANTS AND THEIR FOOD.-III. 



By H. H. W. Pearson, m.a. 



Carbon, the most abundant constituent of the plant's 

 food, is drawn from the atmosphere in the form of 

 Carbon dioxide.* Equa lly necessary, however, are the 



• Knowledge, March, 1900. 



mineral elements supplied by mother earth. Of these 

 the following are indispensable to most green plants — 

 Phosphorus, Sulphur, Potiissium, Magnesium, Calcium 

 and Iron; further investigation may show that others 

 are also essential in certain cases. For the comparatively 

 few plants whose food requirements have been carefully 

 examined these elements comprise all the mineral food 

 requisite to support vigorous growth. Nevertheless, 

 otliers, although they appear to play no direct role iu 

 the process of nutrition, are also absorbi'd. 



These elements are stored iu boundless quantities in 

 the rocks of the earth's surface. In the ordinary pro- 

 cesses of Nature the rocks are disintegrated and their 

 fragments contribute to the formation of a loose carpet 

 in which the roots of {plants grow and extract therefrom 

 a portion of their food. This is the soil, consisting 

 usually of a mixture of rock-particles, disintegrated and 

 partly decomposetl, with humus (vegetable mould), an 

 organic substance resulting from the partial decay of 

 the dead botlics of plants and animals. This summary 

 definition of soil is, be it noted, very far from conveying 

 an adequate idea of its exceedingly varied structure 

 and highly complicated nature. It is only quite recently 

 that any serious attempt has been made to grapple with 

 the enormous difiicultics confronting the investigation 

 of its structure and properties, with which the supply of 

 plant food is very closely connected ; until they are 

 better understood our knowledge of the mineral food 

 supply of plants must remain very imperfect. The 

 chemical and physical properties of the soil are now 

 being cai-efully studied, particulai-ly in the United 

 States ; in this country and in Germany much has of late 

 been discovered concerning the work of living organisms 

 in the soil, more directly in connection with the prepara- 

 tion of the compounds from which Nitrogen, one of the 

 most important constituents of the food, is obtained. 



That soils differ from one another in a very marked 

 manner is a fact easily demonstrated. Differences, for 

 example, between the stiff clays of Suffolk, the sands of 

 Bedfordshire, and the Buckinghamshire chalk, cannot 

 be forgotten by one who has walked over them in wet 

 weather. If specimens of these soils are dried and 

 analysed it is found that diversity in chemical com- 

 position is much less than would be expected. The 

 same elements are present in them all, and in pro- 

 portions not varying much from one to the other. The 

 distinctive characters gi'atuitously forced ujJon the notice 

 of the pedestrian ai-e due to their dissimilai- behaviour 

 towards water. This depends upon their physical pro- 

 perties, the relative sizes of the constituent particles, 

 and not the elements the particles contain, for these are 

 the same in all. 



An analysis of a soil shows that it is composed of a 

 great many substances ; some arc of no importance to 

 plants. Those substances a plant demands from the 

 soil it grows in are Potash, Lime, Magnesia, Iron, and 

 Phosphoric and Sulphiu-ic Acids (the latter in the form 

 of phosphates and sulphates respectively). In addition 

 to these, plants also take up others, for examjjle, Silica 

 and Soda, which appear not to form a necessary jjart of 

 the food supply. Silica, indeed — and this is true of 

 many other substances as well — is harmful to the plant 

 if absorbed in too great quantity. The largo quantities 

 of Silica present in the external cells of grasses and 

 some other plants — responsible for the " cutting edge " 

 of many grass-leaves — are really rejected matter de- 

 posited by the plant in the outer cells to be out of the 

 way, this being one of the few methods available to 

 vegetable organisms of disposing of useless solid matter. 



