200 HOW CROPS GROW. 



fibers of various plants, viz., common hemp, Manila 

 hemp (Musa textilis), aloe-hemp (Agave Americana), 

 common flax, and New Zealand flax (Phormium tenax) 

 are incrusted with silica. In jute (Corchorus textilis) 

 some cells are partially incrusted. The cotton fiber is 

 free from silica. Wicke (loc. cit.) suggests that the du- 

 rability of textile fibers is to a degree dependent on their 

 content of silica. 



Sachs, in 1862, was the first to publish evidence that 

 silica is not a necessary ingredient of maize. He ob- 

 tained in his early essays in water-culture a maize plant 

 of considerable development, whose ashes contained but 

 0.7% of silica. Shortly afterwards, Knop produced a 

 maize plant with 140 ripe seeds, and a dry-weight of 50 

 grammes (nearly 2 oz. av.) so free from silica that a 

 mere trace of this substance could be found in the root, 

 but half a milligramme in the stem, and 22 milligrammes 

 in the 15 leaves and sheaths. It was altogether absent 

 from the seeds. The ash of the leaves of this plant thus 

 contained but 0. 54 per cent of silica, and the stem but 

 0.07 per cent. Way & Ogston had found in the ash of 

 field-grown maize, leaf and stem together, 27.98 per 

 cent of silica. 



In the numerous experiments that have been made 

 more recently upon the growth of plants in aqueous solu- 

 tions, by Sachs, Knop, Nobbe & Siegert, Stohmann, 

 Rautenberg & Kiihn, Birner & Lucanus, Leydhecker, 

 Wolff, and Hampe, silica, in nearly all cases, has been 

 excluded, so far as it is possible to do so, in the use of 

 glass vessels. This has been done without prejudice to 

 the development of the plants. Nobbe & Siegert and 

 Wolff especially have succeeded in producing buckwheat, 

 maize, and the oat, in full perfection of size and parts, 

 with this exclusion of silica. 



Wolff ( Vs. St., VIII, p. 200) obtained in the ash of 

 maize thus cultivated, 2 to 3% of silica, while the same 



