4 CHEMISTRY OF PLANTS. 



The ashes of Equisetum limosum yielded 94-85 per cent of silica 

 Equisetum urvense . . 9.5.48 



Equisetum hyemale . . 97 '52 ,, 



Calamus Rotang . . 97*20* 



Where the silica is in very large quantities, as in the bark and epi- 

 dermis of the larger grasses, the tubular palms, and the equisetums, the 

 ashes by careful burning may be made to retain the form of the plant so 

 accurately, that even microscopic organs may be readily distinguished. f 

 The silica in these plants exists in the form of small plates, grains, or 

 needles, which are often melted together by the heat ; but if the part of 

 the plant is submitted to the action of sulphuric acid, the silica retains 

 its primitive forms, This proves that the silicium is not chemically 

 united with the tissue of the plant, as is stated by ReadeJ ; or even, in- 

 deed, organised, as was formerly gratuitously maintained. 



Potassium, Sodium, Calcium, Magnesium, Aluminium, Iron, Man- 

 ganese, and Copper, are present only as oxides in combination with acids. 

 The first seven exist in very varying proportions in most plants : copper 

 probably in only a few. There is an old saying among the people, 

 especially in the north of Germany, that the wood of the lime contains 

 gold. 



On the origin of these substances in plants, and more particularly 

 with regard to the question as to whether plants take them up from the 

 earth, or form them by a peculiar process of vegetation out of the first 

 four elements above-named, there is but one opinion amongst chemists 

 and physiologists, and that is, that no elementary body can be present in 

 plants that has not been taken up from without the plant. The opposite 

 view, maintained by Reade||, can only be regarded at the present day as a 

 curiosity, and scarcely deserves reference but for the refutation supplied 

 by the labours of Saussure, Davy, Lassaigne, John, Jablonsky% and 

 others. It is difficult also to divine what could have induced the Berlin 

 Academy to give its prize to the single rough experiment made by 

 Schroder, and the confused reasoning of Neumann ; which, supported, 

 indeed, by Braconnot, first brought this absurd view into vogue.** If 

 we consider how small the quantity of solid matter is in most plants, and 

 the large quantity of water they take up and allow to evaporate, we 

 shall have no difficulty in accounting for the presence of substances in 

 plants, which, when diffused through the water absorbed, would resist 

 the test of the most delicate re-agents. 



6. The foregoing elements form amongst themselves certain 

 binary combinations, of which the following are the most important 

 that are met with in plants : 



a. Compounds with Oxygen. Of these, water (HO or H) and 

 carbonic acid come first (CO 2 or C); then oxalic acid (Oor C 2 O 3 ) 

 and the other oxygen acids ; and lastly, the oxides of the metals. 



Water is the most important : without it no chemical change could 

 take place, to say nothing of vital processes. Most plants contain large 



* H. A. Struve de Silica in Plantis nonnulla. Diss. inaug. Berol. 1835. f Ibid. 

 \ London and Edinburgh Phil. Mag. and Journ. 1 837. Nov. 

 See A. v. Humboldt Floras Fribergensis Specimen, Berol. 1793, p. 134. 

 || Op. cit. 



^f Jablonsky de Conditionibus Vegetationi necessariis quaedam. Diss. inaug. Berol. 

 1832. ** Tbid. p 78. 



