170 THE FORMATION 
one is indicated by a, and the others by the first two letters, as an, al. In 
case two species of the same genus are present, the species initial i¢ used in 
connection with that for the genus, as ac and ar for Agropyrum caninum and 
Agropyrum richardsonti respectively. It is rarely necessary to exceed two 
letters for any species. Plants which regularly have several stems from the 
same root are indicated by the initial and an exponent as a*. Seedlings are 
represented by a line drawn through the letter. Usually the chart sheet 
affords sufficient space below the chart for the legend. When the list of 
species is long, the back of the sheet is used. 
210. Factors and photographs. Each chart is numbered, and the forma- 
tion, station, and date indicated. The constant factors, altitude, slope, and 
exposure are ascertained and recorded on the sheet. The variable factors 
are read in each quadrat whenever possible, and in addition to being pre- 
served in ‘the record book, are noted on the chart sheet along with the base 
reading in the formation for the same time. This facilitates the interpreta- 
tion of the differences found when two or more charts are compared. Chart 
quadrats are regularly photographed. For this purpose a long-focus 4 x 5 
camera with a telephoto lens is used. At the proper distance this will make 
a view of the same size as the chart, thus making possible an exact compari- 
son of the two. The chart and photograph serve as mutual checks, as well 
as complements, since the former shows number, position, and arrangement, 
and the latter, height, form, position, and arrangement. The view is usually 
made by placing the camera directly in front of the middle of the lower tape, 
at such a distance that the side tapes fall just within the limits of the ground 
glass. The swing is always used in order that the focus may be uniformly 
sharp. Surface views of the quadrat can be taken by means of a device 
which permits the camera to hang downward from the tripod, or by means 
of a tripod with a swinging platform. Such views are especially valuable 
for the study of competition, since they give a clear idea of the spread and 
density of the various plants. They are difficult to make unless the vege- 
tation is low and nearly uniform in height. The usual photograph is much 
more serviceable in regular quadrat work. 
The Permanent Quadrat 
211. Description and uses. As stated heretofore, either list or chart quad- 
rats may be rendered permanent in order that they may be followed from 
season to season or from year to year. Asa matter of fact, however, an area 
which is to be studied repeatedly really demands charting, and in practice 
chart quadrats alone are made permanent. This is done simply by driving 
