VIII. SUMMARY. 



This study, which is in part a continuation of the investigations re- 

 corded in ' 'E cological Relation? of Roots, ' ' but also includes an investiga- 

 tion of the root development of crop plants, was undertaken with the 

 following aims: (1) to increase our knowledge of the root habits of 

 dominant and subdominant plants growing under a wide range of 

 climatic and edaphic conditions; (2) to determine more fully the root 

 distribution and root competition of the individual species in relation to 

 other species of the community; and (3) to find the root relations of 

 grassland communities as units of vegetation. Further aims were: 

 (4) to determine the relation of the root habits of plants in various com- 

 munities to their successional sequence; (5) to ascertain the correlation 

 of the root development of cereal-crop plants with the different grass- 

 land associations, thus rendering more exact the indicator value of 

 various native species used in classifying land for different types of 

 agricultural practice or for purposes of grazing only ; and finally (6) to 

 obtain some knowledge of the root development of crop plants when 

 grown under different environmental conditions. For the proper 

 setting of these studies "Ecological Relations of Roots" forms the back- 

 ground. 



Investigations have been carried on at more than 25 stations in 

 Colorado, Kansas, Nebraska, and South Dakota, from the Missouri 

 River to the Rocky Mountains. A study of practically all of the grass- 

 land dominants has been completed ; many of them have been examined 

 repeatedly in more than two associations; this, with the study of impor- 

 tant sub dominants, has brought the total number of root systems ex- 

 cavated and examined up to about 1,500, including those of about 150 

 grassland species. More than 80 examinations of the root systems of 

 crop plants have been made in widely varying soil types and conditions 

 of growth. 



The trench or pit method was used in excavating roots, the soil about 

 the root systems being removed with appropriate apparatus from the 

 face of the trench and the roots photographed either in place or against 

 a suitable background. In many cases drawings of the root systems 

 were made to scale as they were uncovered. The quadrat-bisect 

 method has also been employed to show the root systems of communi- 

 ties and of crop plants in place and thus to exhibit their inter-relations 

 in detail. 



The grassland formation, because of pronounced climatic variations, 

 especially in rainfall and evaporation, has been differentiated into sev- 

 eral associations (Clements, 1920), of which three are considered here. 

 True prairie (Stipa-Kceleria association) is characterized by a luxuriant 



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