8 Methods of Working 



nature, a liquid medium replacing the solid one, so that the roots have 

 free access to every part of the substratum without meeting any opposi- 

 tion to their spread until the walls of the culture vessel are reached. The 

 conditions of aeration are also different, for while the plant roots meet 

 with gaseous air in the interstices of the soil, in water cultures they are 

 dependent upon the air dissolved in the solution, so that respiration takes 

 place under unusual conditions. It is possible that the poverty of the 

 air supply can be overcome by regular aeration of the solution, resulting 

 in decided improvement in growth, as L. M. Underwood (1913) has shown 

 in recent work on barley in which continued aeration was carried out. 



2. Scmd cultures. 



This method has the advantage over water cultures in that the 

 environment of the plant roots is somewhat more natural, but on the 

 other hand the work is cumbersome and costly, while the conditions 

 of nutrition, watering, &c., are less under control than in the water 

 cultures. Sand cultures represent an attempt to combine the advantages 

 of both soil and water cultures, without their respective disadvantages. 

 Generally speaking perfectly clean sand is used varying in coarseness 

 in different tests, and this is impregnated with nutritive solutions 

 suitable for plant growth. The sand is practically insoluble and sets 

 up no chemical interaction with the nutritive compounds, while it 

 provides a medium for the growth of the plant roots which approxi- 

 mates somewhat to a natural soil. It is probable, however, that a 

 certain amount of adsorption or withdrawal from solution occurs, 

 whereby a certain proportion of the food salts are affiliated, so to speak, 

 to the sand particles and are so held that they are removed from the 

 nutritive solution in the interspaces and are not available for plant 

 food, the nutritive solution being thus weakened. The same remark 

 applies to the poisons that are added, so that the concentration of the 

 toxic substance used in the experiment does not necessarily indicate 

 the concentration in which it is presented to the plant roots. On the 

 other hand, undue concentration of the solution is apt to occur on 

 account of the excessive evaporation from the surface of the sand. The 

 sand particles are relatively so coarse in comparison with soil particles 

 that the water is held loosely and so is easily lost by evaporation, thus 

 concentrating the solution at the surface, a condition that does not 

 apply in soil work. With care this disadvantage is easily overcome as 

 it is possible to weigh the pots regularly and to make up the evaporation 

 loss by the addition of water. 



