1 6 INTRODUCTORY [chaP. 



place, however favourable the other conditions are. 

 Temperature, the supply of water, the proportion of 

 carbonic acid in the air, the number and area of the 

 stomatic openings, are all limiting factors of this kind, 

 as also is the supply of other nutriment to the plant. 



Though the compounds of carbon with hydrogen 

 and oxygen make up so much of the solid matter of 

 the plant, the remaining substances, comparatively 

 small in amount as they are, are still all-important to 

 the process of growth. The part they respectively play 

 and their mode of entry can best be illustrated by the 

 method of water cultures, of which Fig. i shows an 

 example. By this method the roots of young seedling 

 plants are just allowed to dip into a large jar of water 

 in which salts of the elements found in the plant are 

 dissolved. A complete solution might be made up as 

 follows : — 



Grammes per litre. 



Calcium Nitrate 07 



Potassium Phosphate o-6 



Potassium Chloride ..... o-8 

 Magnesium Sulphate 0-3 



with a trace of ferric chloride. 



This will contain all the elements, except silicon, 

 normally found in plant ashes, and under such condi- 

 tions the plant will grow and go through its whole 

 cycle of life, assimilating freely, producing large quanti- 

 ties of dry matter, setting flowers, and ripening healthy 

 seed. Certain precautions have to be taken, but if the 

 right conditions are assured, the growth of a plant in 

 a water culture is perfectly normal, and may be taken, 

 as far as the plant is concerned, as representing the 

 course of its nutrition in the field. The advantage of 

 the method lies in the fact that it is possible to vary 

 the composition of the nutrient solution by omitting in 

 turn from successive jars each of the salts used in 



