iqiq] waterman— root SYSTEMS 51 



2. The study of the habitat selected shows that dune sand as a 

 substratum for plant growth is almost unique in uniformity of 

 texture and in absence of mineral salts required by growing plants. 

 It is homogeneous chemically, but contains not only old soil layers 

 but minute streaks and patches, apparently of carbonaceous and 

 organic origin, as well as dead plant parts, very unequally dis- 

 tributed. 



3. The roots of dune species react differently to the elements of 

 this heterogeneous structure, extension being increased in some 

 species by the buried organic matter, while others seem unaffected 

 or even inhibited by it. 



4. These reactions are specific and hereditary, and may reflect 

 the conditions under which the ancestral plants grew. They must 

 be regarded as of great importance in the choosing of species for 

 introduction into conditions where the humous content is uneven. 



5. Giving due weight to the possibiUty of moisture, oxygen 

 content, and penetrabiHty of the sand as influencing factors, the 

 evidence seems to point conclusively to nutrient or at least chemical 

 influence as the cause of variability in symmetry in the extension 

 of roots under dune conditions. 



6. Under certain conditions the root apparently utilizes organic 



matter directly, at the expense of its shoots. Extreme lengthening 



and thickening of roots occurring under these conditions call into 



question the value of the common method of estimating plant 



growth by measuring the length and weight of roots. 



Northwestern' University 

 EvANSTON", III. 



LITERATURE CITED 



1. Alten, H. von, Wurzelstudien. Bot. Zeit. 67:175-199. 1909. 



2. Benecke, W., tJber die Keimung der Brutknospen von Limaria cruciata. 

 Bot. Zeit. 61:19-46. 1903. 



3. Bennett, M. E.. Are roots aerotropic? Bot. Gaz. 37:241-259. 1904. 



4. BusGEN, M., Studien iiber die Wurzelsysteme einigen dicotyler Holz- 

 pflaazen. Flora 95:58-94. 1905. 



5. , Bau und Leben unserer Waldbaiime. Jena. 1897. 



6. Cannon, W. A., Root habits of desert plants. Carnegie Publ. 131. 191 1. 



7. , Treelessness in prairie regions. Carnegie Yearbook 12:71, 72. 



1913- 



