CARTOGRAPHY I83 



in order that the segments of successive years may exactly correspond. 

 The record sheet is labeled, dated, and filed. By folding at one edge, it may 

 be filed in the regular field book. 



228. The denuded circle is established in the same way as a permanent 

 one. The original position of the individuals of the species under considera- 

 tion may be recorded or not, depending upon the use to be made of the 

 results. The safest plan is first to read the circle in the usual way, and 

 then to denude it. The latter should be done in such a way as to remove 

 all the disseminules from the surface in so far as possible. It is essential 

 also that tliis be done before the seeds are mature and begin to be scattered. 

 The central plant or cluster is of course not removed. In special cases, all 

 the plants of the species are allowed to remain to serve as centers of coloniza- 

 tion. The successive yearly readings of the denuded area are made exactly 

 as for a permanent circle. Permanent and denuded circles, like quadrats, 

 should always be established near each other so that they permit of ready 

 comparison under similar conditions. 



229. Photographs of migration circles furnish the most detail when 

 the camera is placed just behind the central group in such a way as to show 

 its relation to the other individuals or clusters of the circle. In the denuded 

 circle, or when the plants stand out conspicuously from the bulk of the 

 vegetation, it is not necessary to use guidons, but in other cases the latter 

 greatly increase the value of the picture. Factor readings are less im- 

 portant for migration circles than for quadrats and transects. The factors 

 of principal importance are those that deal with migration and ecesis, i. e., 

 wind, water-content, and soil temperatures. The former may be determined 

 for both circles in common, but the conditions that affect ecesis must be 

 observed separately for each. 



CARTOGRAPHY 



230. Value of cartographic methods. Chart, map, and photograph are 

 records indispensable to the systematic study of vegetation. They serve 

 not merely to preserve the facts ascertained, and to permit their ready com- 

 parison, but they also put a premium upon accurate methods, and conse- 

 quently bring to light many points otherwise overlooked. For ecology, 

 they have the value which drawings possess in taxonomy, in that they 

 make clear at a glance what pages of description fail to indicate. They 

 are the fundamental material of comparative phytogeography, and in all 

 careful vegetational study their use is no longer optional but obligatory. 



