288 LECTURE XVII. 



and some compound of iron, are found in every soil in which plants grow, and in 

 the water of rivers, wells, and seas, and are undoubtedly used by the plants. 



Mere chemical analysis, by which the presence of the materials in plants 

 is proved, would by no means alone suflfice to show whether all the substances 

 detected by the analysis are really indispensable for the nourishment of the plant. 

 As a matter of fact, numerous other elements in addition to those mentioned 

 have been discovered now and then in plants, occasionally indeed in considerable 

 quantity. I need only mention iodine in marine j)lants, manganese in many land- 

 plants, and the very considerable quantities of zinc in plants which grow on 

 calamine soils, but which in other places flourish just as well without zinc. One 

 of the commonest phenomena of this kind, however, is the accumulation of silica in 

 the tissues of very many plants, although we may regard it as certain that this 

 substance is superfluous for the chemical processes of nutrition, as well as for the mole- 

 cular movements connected with growth. Among the plants in the epidermis of which 

 particularly large quantities of silica are deposited, we find, besides the Equisetacese 

 and some others, the Grasses, and among these the Maize. Although in such plants 

 half of the total ash often consists of silica, I (and subsequently other observers 

 also) nevertheless succeeded in bringing Maize plants to vigorous and complete 

 development with the help of nutritive solutions to which not even a trace of silica was 

 added ^ ; the small traces of silica detected with difiiculty in these experimental plants 

 could only have been derived from the glass walls of the vessels, or from the dust. 

 Although it is thus shown that this substance, widely distributed in plants, is 

 superfluous for the purposes of nutrition and growth, it would be going too far 

 to regard it as superfluous also in every other connection. The silica is generally 

 deposited in the outer walls of the cells of the epidermis of the leaves and stems, 

 i. e. of the transpiring organs : it is conveyed there with the water of nutrition, and, on 

 the evaporation of this, remains behind in the outer cell-wall. By burning the latter 

 on platinum foil, especially with the addition of sulphuric acid, the silica is obtained 

 in the form of so-called skeletons, that is, thin plates which still possess with great 

 exactness all the finer structure of the epidermis cell-walls, and especially of the 

 hairs. The form of these skeletons shows that the molecules of silica have been 

 regularly deposited in the cell-walls, between the molecules or micellae of cellulose ; 

 the walls of vessels, wood-cells, and parenchyma cells in old leaves (e.g. of Oaks 

 and Beeches in the autumn), moreover, aff"ord exquisite siliceous skeletons. Among 

 the most remarkable phenomena of this kind is the deposition of silica in the cell- 

 walls of the unicellular Algae of the subdivision of the Diatomaceae, of the siliceous 

 skeletons of which the well-known silica deposit of Bilin in Bohemia, for example, 

 consists. It is scarcely probable that these Algae could flourish in a medium devoid of 

 silica, and whether the Equisetums and Grasses which abound in silica would maintain 

 themselves permanently if the supply of silica were cut off is more than questionable, 

 although it is established that this substance is superfluous for the processes of 

 nutrition and growth, and quite certain that the exceedingly fine masses of silica 

 in the epidermis of these plants scarcely at all afiects the elasticity and rigidity 



' Cf. my treatise in ' Anitalcii der Landwirthschaft in den kgl. preuss. Staaten,' Wochenblatt, 

 t862, p. 1S4. 



