104 HISTOLOGY AND PHYSIOLOGY. 



soil-particles. If a polished granite, or marble slab, be 

 placed under the soil, exposed to the action of the rootlets, 

 and then examined at the end of the growing season, it 

 will be found to be sensibly roughened and dissolved 

 wherever the rootlets came in contact with it. 



FOOD-ELEMENTS. 



132. The food-elements, which plants consume, may 

 be determined in two ways: (1) by chemical analysis of 

 vegetable tissue ; or (2) by causing plants to grow in pure 

 water, to which are added the compounds containing the 

 elements that will nourish them. They have been found 

 to be carbon, hydrogen, oxygen, nitrogen, sulphur, 

 iron, and potassium. If all these are present, the plant 

 will grow ; if any one or more are absent, the plant will 

 die of starvation. Besides these seven elements, plants 

 appropriate phosphorous, calcium, sodium, magne- 

 sium, chlorine, and silica. These six are of secondary 

 importance, for without them the plant may grow. In 

 marine plants, iodine and bromine are always present. 

 In special cases are found also at times small quantities 

 of aluminum, copper, zinc, lithium, manganese, 

 nickel, cobalt, strontium, and barium. Calcic 

 fluoride is contained in the bones of animals; and, as 

 their food is furnished wholly by plants, the presence of 

 fluorine in the latter is inferred. 



133. Oxygen may enter the plant in a free or uncom- 

 bined state. All the other elements are absorbed in the 

 condition of compounds, namely, water, carbonic dioxide, 

 and the nitrates, sulphates, carbonates, phosphates, silicates, 

 and chlorides of ammonia, potash, lime, iron, soda, and 

 magnesia. Carbon constitutes usually about one-half of 

 the entire dried substance of the plant. Yet this large 



