54 



SUMMARY OF CONCLUSIONS. 



The one conclusion from these experiments which 

 may be regarded as satisfactorily certain- is that differ- 

 ent species of plants differ in the oxygen supply neces- 

 sary for their roots. It follows that there are differ- 

 ences in the response of different species to different 

 degrees of soil aeration and that these differences 

 are probably important ecologically and agriculturally. 



It is a reasonable inference from the data 

 but is not fully certain that the injuries worked in 

 certain plants by deficiency of oxygen in the soil 

 are due to interferences with the respiration of the 

 root protoplasm. In Coleus this seems to affect first 

 the water-absorbing power of the roots. This is probably 

 not true of the other species tested. 



The two experiments with carbon dioxide indicate 

 that this gas has a specific poisonous effect on Coleus 

 but none on willow. 



