98 



CASSELL'S POPULAR GAEDEXIXG. 



in the plant dv far the larger proportion consists of 

 carbon, and this is obtained from the atmosphere 

 through the agency of the leaves, and may conse- 

 quently be passed over till the subject of leaf -action 

 is dealt with. The other solid matters of which a 

 plant consists are derived from the soil by the 

 agency of the roots. These arc earthy or mineral 

 substances, destitute of nitrogen, or in other cases 

 containing that important element ot plant con- 

 stitution. B}' chemical analysis it has been found 

 that a large number of mineral or niti'ogenous 

 substances exist in plants. Analysis also has shown 

 that some of these ingredients exist in much larger 

 quantities than others in the same plant, and that 

 the quantity of any particular ingredient in the 

 plant varies with the age and state of growth. 

 Some parts of the same plant may contain much, 

 others none at all. Again, while one plant may 

 contain much of one substance, another may liiive 

 little or none, and this even although the plants 

 may be growing side by side under the same 

 conditions, and even although tlicy may be very 

 nearly allied plants. Two grasses for instance, 

 species of Foa, growing together, and so much alike 

 that only the practised eye can discriminate them, 

 have yet a very different chemical constitution so far 

 as these mineral substances are concerned. Chemical 

 analysis therefore rep e als the existence of numerous 

 mineral ingi-edients in the plant, and in varying 

 proportions : a similar analysis of the soil shows the 

 source whence they are derived. Analysis, however, 

 does not tell us of wliat use these matters are in 

 the plant, nor what precise proportion of them is 

 requisite. 



Essentials of Plant Dietary. — It is a com- 

 mon thing to include all the substances found 

 in plants by analysis as plant-foods, and in text- 

 books an imposing array of chemical substances 

 is dirly catalogued under the heading '-food 

 of plants." The heading is misleading in several 

 ways : it leads the readei- to form an exagge- 

 rated idea of their value, and practically to under- 

 value, if not to ignore, such infinitely more im- 

 portant matters as water and carbonic acid gas. It 

 induces him to consider them, as enumerated, as so 

 many articles of food ; the truth being that as such 

 they are none of them foods : and, moreover, that 

 only some of them really enter into the composition 

 of plant-food, and those in small and variable 

 proportions. These errors arising from the unchecked 

 use of chemical analysis may be eliminated by the 

 study of the practical results of experiment on the 

 growth of plants, not by asking the chemist to tell 

 us what remains in his vessels after their contents 

 have been subjected to fire or to destructive analysis, 



but by actually interrogating i\e plant itself as it is 

 growing, and by making it tell us what it is doing at 

 a particular time, and what substances are requisite 

 to enable it to do so. 



This is effected experimentallj' by growing the 

 same plant under various conditions, with a large or 

 with a smaU x)rox)ortion of one particular substance, 

 alone or in combination, or in other cases with none 

 at all. These procedures are constructive, rather 

 than destructive like chemical analysis, and they 

 illustrate the desirability in all cases of establishing 

 trial-groimds, or of setting aside part of the garden 

 or even a few flower-pots for experimental jjurposes. 

 By such means it has become possible to eliminate 

 from the large quantities of various substances found 

 in plants, those that are essential and those that 

 <ire non-essential, and to ascertain what j)roportions 

 of each particid.ar ingredient are most favourable 

 at various stages of growth and development. 



The essential mineral ingredients found in the 

 ash of plants, after birrning away the more perish- 

 able matters, are potassium, the basis of potash ; 

 magnesium, the basis of magnesia; calcium, which 

 stands in like relation to the salts of lime ; iron : 

 phosphorus : and a little sulphur. These, witli 

 water, nitrogen, or ammonia, and carbonic acid, con- 

 sisting of oxygen and carbon, constitute the food of 

 l)lants. or rather the raw materials out of which the 

 food is made. 



Adverting to the mineral matters which remain in 

 the ash of jilants after burning, we have now to in- 

 quire in what form and in what manner they gain 

 entrance into the plant. It is certain they do not 

 enter it as potassium, magnesium, or iron, as the case 

 may be. Again, it is certain that they do not and 

 cannot enter the plant in a solid state, and are 

 not found in the liAing plant except in combina- 

 tion with other substances. Thus the potassium 

 occurs in combination with oxygen, and with cer- 

 tain acids formed in and by the j)lant itself, sucli 

 as oxalic, mahc, citric acids, (fcc. In like manner 

 the ii'on exists in the form of salts ; the phosphorus 

 is imited with oxygen, forming phosphoric acid, and 

 this again is united with various bases to form salts. 

 The full history of all these matters belongs to 

 chemistry rather than to i)hysiology, so that we 

 can only offer here a few considerations as to the 

 manner in which these salts gain access to the plant. 

 In the first place, it is necessary to point out that, as 

 a riile, subject to a few exceptions which we need not 

 stop to consider, these salts do not exist as such in 

 the soil. 



Soils. — It is found by careful experiment, as well 

 as by the coarser operations of tillage and drainage, 

 that the mechanical nature of the sell, and its power 



