14 WATER-EELATION BETWEEN PLANT AND SOIL. 



pletion of this series the cultures were continued, generally with daily 

 determinations, till April 29, after which date weighings and readings 

 were discontinued. 



During the latter part of the period following the second series of 

 houriy measurements, the plants were considerably injured by inade- 

 quate supply of water. If such cultures are to be operated with such 

 intense evaporation as was experienced by these plants in April, it 

 appears desirable to decrease the resistance to water absorption by the 

 plants. Probably this might be accomplished by employing more 

 than one auto-irrigator cup per culture. A shorter water column 

 would have some influence in this direction when Ughter soils were 

 employed, and a lighter (more sandy) soil would in itself be effective 

 in the same direction. The Pelargonium plants appeared to withstand 

 the conditions generally better than those of the other forms. In 

 spite of obvious conditions of drought, however, several perfect seeds 

 were finally harvested from the Vicia plants and the others made very 

 satisfactory growth. It should be noted here that the roots of the 

 Coleus plants were found, at the end of the experiments, to be badly 

 infested with nematodes. 



FLUCTUATION IN SOIL-MOISTURE CONTENT WHEN THE AUTO- 

 IRRIGATOR IS EMPLOYED. 



The value of the auto-irrigator in maintaining soil-moisture content 

 with but small fluctuations has been emphasized in earUer papers,^ but 

 we are not aware of any published data upon the actual degree of 

 fluctuation which is to be expected in cultures equipped with the 

 device. Our data furnish considerable evidence in this connection, 

 and we digress here in order to present this evidence. 



There were but two possible causes for considerable alteration in 

 the weight of oiu- cultures : variation in soil-moistiire content and vari- 

 ation in plant weight. No means is yet at hand for distinguishing the 

 effects of these two possible variations, and it is assumed in our calcula- 

 tions that the weight of the plants did not vary. We shall deal here only 

 with the two periods of hourly observation, each period extending over 

 about a day, so that fluctuation in the amount of non-aqueous material 

 within the plants is easily negligible. Fluctuation in the water content 

 of the plants of a culture during 24 hours is surely not so insignificant, 

 but lack of methods for its separate study has forced us to the assump- 

 tion just stated. Moreover, the plant weight employed in our calcu- 

 lations is, in each case, not the actual weight of the plants concerned, 

 either at the beginning or end of the day period in question, as ought 

 to be the case, but it is the weight exhibited by the plants at the end 

 of the whole series of observations, when the cultures were discontinued. 



'Livingston, 1908; Hawkins, 1910; Transeau, 1911, as previously cited. 



