144 Plant Physiology 



These seven elements, in addition to hydrogen, oxygen, 

 and carbon (this last supplied by the carbon dioxid of the 

 air, see Chapter IX), are those indispensable for green 

 plants generally ; and the absence of any one of the seven 

 in the nutrient solution will eventually result in the ces- 

 sation of growth. 



In the preparation of the cultures it is convenient to 

 employ as culture vessels ordinary glass tumblers (Fig. 44) 

 covered with black paraffined paper, preferably doubled; 

 a shell of black paper is also fitted over the remainder of 

 the tumbler, and wire guards to assist in supporting the 

 plants as they grow are attached with rubber bands. 

 Canada field peas (Pisum arvense) give a quick growth, 

 and are satisfactory for this work. They have the dis- 

 advantage of being unusually sensitive to a lack of calcium, 

 as discussed later. Wheat or oats may be used, and these 

 do well, especially when the solution is often renewed. 

 In general, the cereals in solution culture respond quickly 

 at the outset to potassium and to nitrogen, and, relatively 

 speaking, there is often a deficiency of these elements in 

 the seed. 



Solution cultures in vessels of the size above noted are 

 important merely for those observations extending over 

 comparatively short periods. Large vessels of the nature 

 of battery jars, permitting the use of several liters of the 

 solutions, are required when it is desirable to bring the 

 plants to an advanced state of growth, or to maturity. 



78. Nutrient solutions and water cultures. The nu- 

 trient solution may be variously constituted. It must 

 contain the elements previously mentioned, and it may be 

 well to include also sodium and chlorine. It is probable 



