THE FOOD OF PLANTS. 83 



mode of procedure has been dealt with at length in 1, and we 

 ro \v maize plants according to the instructions there given in a 

 complete food solution, one therefore which contains Calcium 

 nitrate, Potassium chloride, Magnesium sulphate, Potassium 

 phosphate, and some Ferric chloride. But simultaneously we also 

 attempt to bring to maturity a maize plant, germinated in saw- 

 dust, whose roots are growing merely in distilled water. This 

 latter plant soon ceases to grow, while those whose roots are 

 growing in a complete food solution continue to develop vigor- 

 ously. In my experiments, the maize plants with distilled water 

 at their disposal unfolded only four leaves ; their growth then 

 ceased. Mineral substances are thus necessary for normal develop- 

 ment. If the roots of plants are surrounded by distilled water 

 and not by a food solution, their growth comes to a standstill when 

 the reserve materials of the seed are exhausted. 



Our food solution is free from soda and silica. The experiment 

 therefore teaches further that neither Sodium nor Silicon is to be 

 considered as an essential constituent of the food. It is, however, 

 possible that for some organisms Silicon is necessary, as e.g. in 

 the case of Diatoms, the microscopical brown organisms so often 

 met with on plants or stones in water, and whose membranes are 

 impregnated with large quantities of silica. In the case of 

 grasses silica may perhaps be spared, but at the same time it is 

 useful, and this may also be the case with the Equisetums. At 

 any rate the amount of silica in the walls of the epidermal cells 

 of the latter is very considerable, as we may satisfy ourselves as 

 follows. 



If we examine surface sections of the stem of Equisetum 

 arvonse we find that strips bearing stomata and strips devoid of 

 stomata alternate with each other. The former, owing to the 

 layer of chlorophyll-containing cells below them, appear green ; 

 the latter colourless. The epidermal cells are longitudinally 

 elongated ; the structure of the stomatal apparatus is a compli- 

 cated one. We lay a surface section on a small plate of mica, 

 treat the section with concentrated Sulphuric acid, and strongly 

 heat it in a gas or spirit flame. We then lay the mica plate, with 

 the ash, on a slide, add a drop of water, and cover the preparation 

 with a cover glass. On microscopic examination we perceive a 

 silicious skeleton which still allows much of the essential struc- 

 ture of the original section to be made out. 



