436 THE FOOD OF PLANTS 



and it may even aid in metabolism, for Kreuzhage and Wolff 1 found that oats 

 fruited badly when grown in nutrient solutions free from silica. Similarly the presence 

 of silica seems to economize the other ash constituents, while the hardness and 

 brittleness which it confers may act as a protection against the ravages of animals 

 and the penetration of fungi 2 . The deposition of silica does not seem to increase 

 the strength and rigidity of the cell-walls, nor is the laying of crops after heavy rain 

 due to the lack of silica, but rather to the partial etiolation of the basal portions of 

 the stems of thickly-sown plants. Hence laying may be avoided by the use of the 

 drill when the grain is sown. Cereals form haulms of normal strength when grown 

 in the absence of silica, and Pierre found that laid crops actually contained more 

 silica than normal ones s . 



As regards the other non-essential elements, only their presence in the ash or 

 the fact of their absorption has been determined, and since no physiological 

 function has as yet been ascribed to any of them, it will suffice to mention a few 

 points of general interest. 



Zinc appears, according to Risse, to be present in all plants which grow 

 upon a soil rich in this metal, as, for example, at Altenberg near Aix-la-Chapelle, 

 where more than 20 per cent, may be present in places. In many such plants 

 a large amount of zinc accumulates, for Risse found that the roots of Thlaspi 

 alpestre contained in 100 parts by dry weight 0-167 parts of zinc oxide, or 1-66 per 

 cent, of the total ash, the stem contained 0-385 or 3-28 per cent, of the ash, and 

 the leaves 1-50 or 13-12 per cent, of the ash. Viola tricolor^ Armeria vu/garis, 

 Silene inflata, &c., were also rich in zinc, which, like calcium, was usually very 

 abundant in the leaves. Zinc is absent from most soils, and when but a trace 

 is present no marked absorption seems to occur 4 . Zinc salts can be presented 

 to the plant only in extreme dilution, for Baumann found that a watery solution 

 containing more than 5 milligrammes of zinc sulphate to the litre soon acted 

 injuriously upon Phanerogams. The zinc is probably deposited chiefly in the 

 cell-walls and in some form which exercises no poisonous effect upon the 

 protoplasts. 



According to Hoffmann 5 the calamine violet ( Viola lutea var. multicaulis) does 

 not alter when grown in soil free from zinc, nor does cultivation upon soil rich in 

 this metal produce any morphological effect upon Viola tricolor and Thlaspi alpestre. 

 Risse observed an especially active growth in plants of Silena inflata and Armeria 



1 Versuchsst., 1884, Bd. xxx, p. 161. 



' Animals: Stahl, Pflanzen n. Schnecken, 1888, p. 72; Kohl, I.e., p. 304. Fungi: Johnson, 

 Wie die Feldfriichte wachsen, Liebig, 1872, p. 205. 



3 Pierre, Compt. rend., 1866, T. LXIII, p. 374 ; L. Koch, Landw. Centralbl., 1872, Bd. II, p. 202. 

 Sorauer (Bot. Jahresb., 1873, p. 521) and C. Kraus (Forsch. a. d. Geb. d. Agr.-Phys., 1890, 

 Bd. xill, p. 252, and 1891, Bd. xiv, p. 59) have shown that laying may be induced by other causes 

 as well. 



* Cf. literature on absorption of Zn and its presence in plants, given by Baumann, Versuchsst., 

 1885, Bd. xxxi, p. i. Risse (Sachs' Exp.-physiol., 1865, p. 153) and Jensch (Chem. Centralbl., 

 1894, i, p. 281) observed a similar absorption of Zn from metallic soils in Upper Silesia. 



5 H. Hoffmann, Bot. Zeitung, 1875, p. 628 ; Unters. iiber Variation, 1877, p. 36. The calamine 

 violet is found in other districts as well. 



