8o PHYSIOLOGY OF NUTRITION 



(Graminese) can develop without this element. The lodging of grain (when the 

 plants fail to stand erect in the field) , which was earlier ascribed to a deficiency 

 of silicic acid (H2Si03) in the soil, is a result of insufficient illumination. This, 

 in turn, is due to too thick planting. Anatomical study' of the stalks of lodged 

 grain shows that they have all the characteristics of etiolated stems (Fig. 51). 

 In healthy stems we find small, thick-walled cells, while in etiolated stalks, 

 whether lodged or nol , the cells are very large and have much thinner walls. 



In laboratory experiments, where plants are protected from some of the 

 unfavorable conditions of the field, silicon is not essential, but this is not true 

 when plants develop under natural conditions. Here silicon appears to play a 

 very important r61e, protecting the plant from attacks of various parasites. 

 Fungus hyphae cannot easily penetrate cell walls that are impregnated with 

 silica. Wheat, rye, etc., grown in nutrient solutions deficient in silicic acid 

 often suffer so severely from rust that only great care can prevent their complete 

 destruction. The hardness of silicated cell walls is also a very good protection 

 against animal attack. Thus, for instance, one plant of Lithospermum arvense, 

 grown in a nutrient solution without silicic acid, suffered severely from plant- 

 lice even though these were removed daily, while two similar plants, standing 

 near by and grown in similar solutions but not tended so carefully, were 

 completely killed by these insects. 



The distribution of siUcic acid in different parts of seeds^ is another indi- 

 cation of its protective action. Millet seeds without the seed-coats contain 

 only from 4.8 to 7.1 per cent, of the total silicic acid of the seed, all the re- 

 mainder (from 92.6 to 95.1 per cent.) being deposited in the seed-coats. Such 

 a marked accumulation of silicic acid in the seed-coats suggests the impor- 

 tance of this substance to plants growing under natural conditions. The 

 investigations of Sabanin upon ripening seeds of millet show that this plant 

 hastens, as it were, to accumulate enough silicic acid in the peripheral parts of 

 the grain (as in the palea) to protect the increasing reserve material from 

 unfavorable external conditions. 



Most plants can live without chlorine, but buckwheat deprived of this 

 element did not attain complete development in Nobbe's experiments, and it 

 was his opinion that chlorine favors the translocation of carbohydrates from 

 the leaves into other organs. Knop, however, obtained normal development 

 of buckwheat plants in a solution without chlorine, so that the question of the 

 r6le of chlorine is stiU unsettled.'^ It is advisable to add chlorine to the nutrient 

 solution when experimenting with plants whose relation to chlorine is not under- 



> Koch, L., Welche abnorme Aenderungen werden durch Beschattung in wachsendeu Pflanzeaorganen 

 hervorgerufen? Landw. Centralbl. Deutschl. 20: 202. 1872. 



2 Sabanin, A. N., Ueber Kieselsiure in den Kfirnern der Hirse (Panicum miliaceum L.) [Abstract in 

 German, pp. 295-302. Text in Russian.] Jour. exp. Landw. 2: 237-302. 1901. 



" Buckwheat has been repeatedly grown to maturity, with production of seed, in water- 

 cultures without any more chlorine than might have been present in spite of aE ordinary pre- 

 cautions to exclude this element, in the Laboratory of Plant Physiology of the Johns Hopkins 

 University; but the possibility remains that the presence of chlorine might produce more 

 vigorous growth. — Ed. 



