CARTOGRAPHY 183 
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 this 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 aré 
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. Thev 
are the fundamental material of comparative phytogeography, and in all 
careful vegetational study their use is no longer optional but obligatory. 
