174 



BOTANY 



essential substances, and, through its capacity to form compounds, as a 

 means of fixing and rendering harmless hurtful by-products. Iron, 



although of the greatest importance in 

 the formation of chlorophyll, is present 

 in plants only in small quantities. 



In order to determine the nutritive 

 value of different substances the method 

 of water-culture has proved particu- 

 larly useful (Fig. 171). In these culture 

 experiments the plants, grown either 

 directly from the seed or from cuttings, 

 are cultivated in distilled water to 

 which have been added certain nutritive 

 salts. If all the essential nutritive salts 

 are present in the culture solution, 

 even larger plants, such as Indian Corn, 

 Beans, etc., will grow to full strength 

 and mature seeds as well as if grown in 

 earth. It is not necessary in these ex- 

 periments to provide carbon compounds 

 in the nutrient solution, as plants do 

 not derive their carbon supply through 

 their roots, but, with the help of their 

 leaves, from the carbonic acid of the 

 atmosphere. 



The young plants would grow for a 

 time just as well in pure distilled water 

 as in the nutrient solution ; but as the 

 supply of nourishment stored in the 

 seeds became exhausted, they would 

 gradually cease to grow, and die. If 

 one of the essential constituents of 

 fig. m.-water-cuitures of Fagopymm p i ant f 00( j ^ e om i t t e d from the nutrient 



esculentum. I. In nutrient solution , . , , , , , 



containing potassium; II., in nutrient Solution, although the yOUllg plants 



solution without potassium. (After would grow better than in the dis- 



Nobbe, reduced.) tiUed ^^ th&J wouM in t j me fee _ 



come abnormally developed. When, for example, a plant is grown 

 in a nutrient solution containing all the essential food elements 

 except iron, the new leaves developed are no longer green, but are of a 

 pale yellow colour ; they are " chlorotic," and not in a condition to 

 decompose the carbonic acid of the atmosphere and nourish the plant. 

 Upon the addition, however, of a mere trace of iron to the solution the 

 chlorotic leaves in a very short time acquire their normal green colour. 



So long as the necessary nutritive substances are provided, the form in which 

 they are offered to the plants, as well as the proportionate strength of the nutrient 



