6 UNIVERSITY OF COLORADO STUDIES 



The importance of local lists is very great in such a geographical area, 

 for they could be used by the student of distribution in working out 

 the large problems in which he is interested. In Colorado, as in any 

 mountainous region, the historical factors such as glaciation, climatic 

 changes and physiographic processes are all of particular consequence. 

 They enter into all discussions of distribution. Every local flora in its 

 composition bears witness to their importance. Hence a series of lists 

 for representative localities throughout the state could be made highly 

 useful. How the plants came to be here, where they came from, why 

 they are found in one locality and not in another — these problems can 

 only be attacked after much more information is gathered in regard to 

 their present-day distribution. 



It is often observed that a given species of plant very common in 

 a certain locality is entirely absent in an adjacent area, although no 

 natural barriers to migration intervene. No doubt often this is to be 

 explained on historical grounds. The species is extending its range from 

 an earlier home and has not thus far reached the area where its absence 

 is noted. In other cases no such explanation can be made, since the 

 identical species is also present at a more distant point. Here, undoubt- 

 edly, the effects of competition with other plants have been important. 

 Just what competition really is and how it works are little known. The 

 student in the field can find here an opportunity for the collection and 

 interpretation of evidence which will lead to interesting conclusions. 



Since the study of elementary species in the genus Crataegus has 

 brought out such an interesting array of forms, a new field in plant 

 taxonomy has been opened to American botanists. Whatever may be 

 thought of the propriety of naming these forms as species, it is certainly 

 desirable that they be known and described and, if possible, their affinities 

 determined. They could then be assembled into such larger aggregates 

 as seemed desirable for purposes of classification. It must not be 

 supposed that Crataegus is the only genus in need of thorough field study. 

 Opulaster and Rosa besides many of the composites offer opportunities 

 for useful work. Indeed, only the most preliminary work on the flora 

 of Colorado has thus far been done and accurate field study in almost 

 any genus will be well worth while. 



