primnrily '); 



/ 'j> 



seem^.to difference in lieht conditions for the two sets of 

 cultures. The 'A'aight of the four-vreek plants under glass is, 

 in most cases, greater than the weight of the plants grown 

 in the open in spite of the higher evaporation rate experienced 

 ty the covered plants. If it is assur:ied that there is a high- 

 er temperature under the glass, the increase in dry weight pro- 

 duced receives a possible explanation. Also, an increase in 

 growth In length of the plants wen placed under glass is ex- 

 actly what would be expected if temperature is high and light 

 intensity low as compared with the values of these factors in 

 the open. The relation between stem height, temperature, and 

 light has heen sufficiently discussed xinder the exposed sta- 

 tions, and will not be repeated here, but it will be seen tha"^ 

 the behavior of the covered plants supports the assumption made 

 as to the relation of this particular growth rate to the en- 

 vironmental conditions here measured. The fact that the leaf 

 area of the covered plants is high in comparison w'th their 

 dry weight may be considered as due to a lowering in amount of 

 dry matter produced b;-. photosynthesis per xuiit of leaf area. 

 Such a lowering might be expected if the light available for 

 photosjmthesis is cut down by interposing between the plant and 

 the light source a screen that absorbs a part of the rays. This 

 explanation of the behaviour of the plants is, however, only an 

 assumption and cannot be -oroved from the data at hand. 



Whatever may be the explanation of the behavior of these 

 plants, the facts as they stand show very clearly that the 

 growth of plants under glass is quite iifferent from growth in 

 the open, and the indication here is that the glass acts on 

 the growth rates directly by screen ing out part of the light. 



