INTRODUCTION 23 



portant for warm-blooded animals. Toxic materials often in 

 very minute amounts retard the growth of plants. One part of 

 the gas ethylene, a constituent of illuminating gas, in ten million 

 parts of air is sufficient to retard the growth of sweet peas and 

 some other plants. The growth of higher animals is also in- 

 fluenced by poisons, but animals are less likely to come in con- 

 tact with them than plants. 



A supply of mineral salts is important for the growth of both 

 plants and animals. Plants require salts of potassium, calcium, 

 iron, magnesium, sulphur, 



phosphorus, and possibly traces 

 of manganese, boron, and 

 zinc. Animals require sodium 



Chloride and Compounds of Figure 13. The effect of temperature on the 



... . growth of wheat. The pots are placed in a 



CalClUm, iron, pOtaSSlUm, mag- trough heated at the left end and cooled 



• j- i 1 with ice at the right end. The temperature 



neSlUm, lOClin, phOSphOrUS, increases, therefore, from right to left. 



sulphur, and possibly others, ^^jj* permissi ° n ° f Henry 

 Compared to the total weight 



of the body the amount required of any one of these elements 

 is very small. All of the iron contained in a corn plant is less 

 than is contained in two ordinary needles, yet the absence of 

 this small quantity of iron completely prevents the growth of 

 the plant. The reason for the importance of some of these 

 mineral elements is fairly completely known. The significance 

 of others is only guessed. 



Light is an important factor affecting the growth of both 

 plants and animals. For green plants light is necessary for the 

 construction of food from the elementary substances carbon di- 

 oxide and water. It also has other effects upon the growth of 

 plants. Everyone has seen the elongated plants with white or 

 yellow color and small leaves which grow in the dark, for ex- 

 ample, the sprouts from a potato tuber. Not so many are ac- 

 quainted with the remarkable effect which the relative length 

 of day and night has upon the differentiation of flowers by 

 plants. Some bloom best when the days are long and the nights 



