Hoagland —118— Plant Nutrition 



subjected to different temperatures and degrees of 

 aeration. Some results are now available, sufficient 

 to indicate that root temperature responses vary pro- 

 foundly depending on the kind of plant studied. (See 

 plate 26). 



Aeration is a factor interrelated with temperature. 

 At higher temperatures the rate of oxidation of carbo- 

 hydrate is increased when abundant oxygen is avail- 

 able to the roots. This may lead to eventual injury. 

 Climatic conditions affecting the growth of the shoot 

 and photosynthesis also modify responses of the plant 

 to varied root temperatures. In experiments with 

 tomato plants developed in artificial culture solutions 

 in the Berkeley greenhouses we have not found it 

 necessary or desirable to heat the nutrient solutions. 

 A favorable air temperature seems to result in suit- 

 able adjustment of the temperature of the nutrient 

 solutions, although it is not unreasonable to suppose 

 that under natural or agricultural conditions the soil 

 may sometimes be too cold early in the growing 

 season to permit rapid root growth and solute ab- 

 sorption. There are practical observations to support 

 this assumption. 



Climatic Factors and Growth of Plants in Nutrient 

 Solutions : — In all this discussion of nutrient solu- 

 tions and their control, I have not yet given enough 

 emphasis to the climatic factors, but you may recall 

 that I said something in the first lecture about cul- 

 tures carried on in controlled chambers. Most of the 

 investigational work has to be conducted in green- 

 houses, without control of light. The climatic factors 

 assume great importance in the study of plants grown 

 to the fruiting stage. Mobilization of inorganic nu- 

 trients in growing tomato fruits profoundly affects 

 the nutrition of the plant and its relation to the 

 culture solution. Nitrogen-carbohydrate relations in- 

 fluencing fruit development depend on light-tempera- 



