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 is used in 

 connection with that for the genus, as ac and ar for Agropyrum caninuin and 

 Agropyrum richardsonii 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 4x5 

 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, positron, 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. De5cription and uses. As stated heretofore, either list or chart quad- 

 rats may be rendered permanent in order that they may l>e followed from 

 season to season or from year to year. As a 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 



