172 THE FORMATION 
wise, the quadrat itself should constitute a station for making factor obser- 
vations. This connection is absolutely essential, since the quadrat is used 
expressly to determine the structural changes, which are produced by physi- 
cal factors, and the reaction of vegetation upon them. Permanent quadrats 
are established in different formations or stages of a succession to trace the 
invasion of new species and the dropping out of old ones in response to com- 
petition. They serve to distinguish the proper formation, which represents 
a particular stage of development, from the mixed formations which precede 
and follow, and also to determine the exact course as well as the rapidity of 
the change that follows each reaction. When applied to different examples 
of the same stage, and to all the different stages of a succession, the whole 
development of the latter may be minutely traced and definitely recorded. 
The importance of following the changes from aspect to aspect is much less, 
since these are periodical rather than dynamic. They are an essential feature 
of structure, however, and it has been the practice to make at least one series 
of aspect charts from each permanent quadrat. 
For tracing the invasion and competition of lichens and mosse’, which play 
a primary role in initial formations, a subquadrat is used. The size varies, 
but it is usually smaller than the quadrat, although the latter is entirely avail- 
able in the case of the large foliose lichens. For the crustose and smaller 
foliose forms, a subquadrat 2 decimeters square is used, and for the larger 
forms and tufted mosses, one of 5 decimeters. In the case of ground forms, 
tapes are employed, and the quadrat is permanently staked. On rocks and 
cliffs, where moss and lichen stages are most common, tapes are impracticable, 
and the quadrat is permanently outlined with paint. Charts of lichen quad- 
rats are made to the usual scale of 10:1. 
212. Manner of use. Permanent quadrats are mapped and photographed 
in exactly the same way as chart quadrats. As soon as this has been done, a 
labeied stake is driven at the upper left-hand corner, so that its edge indicates 
the exact position of the quadrat stake, and a smaller one is placed at the op- 
posite corner to facilitate the task of setting the tapes accurately in later 
readings. The label stake bears merely the number of the quadrat and the 
date when it was first established. It is firmly fixed and allowed to project 
just enough to enable it to be located readily. Its position requires careful 
landmarking when the quadrat is to be visited year by year. In forest for- 
mations, this is readily done by blazing, but in grassland it is necessary to 
have recourse to compass and pacing, or to erect an artificial landmark. 
After several charts have been made, a permanent quadrat attains a high 
value, and every precaution must be taken to prevent losing its exact loca- 
tion. At the second reading of a quadrat, whether in the succeeding aspect 
