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The plant requires a certain definite amount of each essential element. If 

 too little of one element be present the plant is unable to develop healthily, 

 even if the others be present in excess. This fact is sometimes expressed thus : 

 (AD. MAYER, 1902, I, 323) The nutrient present in minimum quantity gives a 

 standard for the amount of production as a whole. (' Law of the minimum.') 



Despite the efforts of numerous investigators we are, unfortunately, still 

 very much in the dark as to wherein the need for these five or six elements lies, 

 and on taking stock of what little we do know we are brought face to face with 

 the further problem as to the combinations in which these elements are employed 

 by the plant. The significance of sulphur and phosphorus has been sufficiently well 

 established, for they are, as we have seen, as essential constituents of proteids 

 as carbon, hydrogen, oxygen, and nitrogen ; sulphur occurs generally in 

 such bodies, but phosphorus only in some of them, e. g. nucleins and many 

 globulins. Further, it is by no means a matter of indifference in what com- 

 binations these elements are presented to the plant ; it has been shown, on 

 the contrary, that they must be presented in highly oxidized forms, such as 

 sulphuric and phosphoric acids. Sulphurous and hyposulphurous are as use- 

 less as phosphorous and hypophosphorous acids, in fact, they are even poisonous 

 to many plants ; nor can sulphur and phosphorus be used in the elemental 

 form. It may be noted, in passing, that only one essential element is absorbed 

 by the plant in the elemental form, viz. oxygen. Further, sulphuric acid must 

 be presented to the plant, united with a metal, although it is apparently a matter 

 of indifference whether it be supplied as a sulphate of potassium, magnesium, 

 or calcium, thus offering simultaneously a necessary metal, or as a sulphate of 

 sodium or aluminium. We are, again, quite ignorant as to the region in the 

 plant where such sulphates and phosphates are transformed or 'assimilated'. 

 SCHIMPER (1890) has shown that plants which absorb abundance of sulphate 

 store it unaltered in many cells ; if, however, only a small quantity of sulphate 

 is available, it is, after entry, altered into a form which no longer gives the 

 sulphuric acid reaction ; no such reaction can be obtained from the young 

 cells of meristem or from buds or pollen-grains. The same is true of phosphoric 

 acid ; it may be presented as a salt of potassium or sodium, and these 

 compounds are more soluble than compounds with calcium, magnesium, \ 

 or iron. In all situations where one fails to obtain the sulphuric acid 

 reaction the phosphoric acid reaction is also wanting ; at the same time, \ 

 certain plants, e. g. the horse-chestnut, Forsythia, the onion, &c., are well 

 known to store up large quantities of phosphate in old parenchymatous cells 

 of the leaf (compare also Lecture XI). In addition to the proteids it may be 

 noted that there are other sulphur-containing compounds in the plant 

 which, however, owing to their limited distribution, need not be referred to 

 here ; they will be discussed later on in Lecture XVIII, where the special 

 significance of sulphur in the vital economy of the sulphur-bacteria is treated of. 



The third non-metallic element we have to consider is chlorine. It is em- 

 ployed only as hydrochloric acid and generally added to the solution as 

 potassium chloride. As already mentioned, chlorine cannot be ranked as of 

 the same importance as sulphur or phosphorus, for although of very general 

 service, it is rarely directly essential. In addition to being present in buckwheat, 

 as already mentioned, chlorine occurs (according to BEYER, 1869) in peas and 

 oats, but as to its special significance in these plants we are entirely in the dark. 

 It might be expected to play some part, more especially in plants which grow 

 in soils containing common salt, but some of these plants develop perfectly well 

 without any chlorine. We are ignorant also whether seaweeds can exist with- 

 out this element. 



Among the metals, potassium is absolutely essential, and it is immaterial with 

 what acid it be united. Efforts have been made to replace potassium by one or 

 other of the related metals, lithium, sodium, rubidium, caesium, but all these are 



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