covered cultures usually shows high values as compared to the 

 corresponding exposed cultures. The tendency noted in previous 

 discussion for this growth rate to fall off relatively, as the 

 plants b-^Gome larger seems to be to a great extent not active 

 here. The consideration of the covered cultures in detail will 

 bring out the features mentioned above. It should be noted 

 that the culture periods for the covered and expased plants 

 correspond to within a day or two. That is, the exposed cul- 

 ture^and covered cultures were started at approximately the 

 same time and thus extend over practically the s?me growth 

 periods. In some cases exigencies of the experiment made it 

 necessary to take measurements on the pl'^nts of the covered 

 cultures on the day prec^eding or the day following the one on 

 which the exposed cultures were cneasured. This, however, would 

 not introduce ancugh of a difference in the measurements to 

 interfere with the general comparisons here made. In the com- 

 parison between the growth for the exposed and. covered cultures, 

 no attempt will b "^ made to account in detail for the differences 

 between the two sets of plants in terms of climatic conditions, 

 since the climatic influences acting on the covered plants 

 are not accutately kno-^i. After the peculiarities of the cov- 

 ered plants have been pointed out, however, an explanation of 

 their general behavior will be given which seems most probable 

 in view of all the facts of the investigation. 



The covered and exposed cultures for Oakland iiffer le3S 

 than the two corresponiing sets at the other stations fBaltimore 

 and Easton) , but show, nevertheless, the general features out- 

 lined above. The plants of the two-week covered cultures for 

 Oakl-?.nd exhibit a much higher value of the leaf-product than 



