46 BASES AND CRITERIA. 



Preliminary results justify the feeling that standard plants or phytometers 

 can be developed with more or less readiness to measure varying amounts of 

 the direct factors, water, light, temperature, soil-air, and solutes. Such func- 

 tional responses as transpiration and photosynthesis furnish the most accurate 

 measurements, but growth responses are also of the greatest value, especially 

 where factor-complexes are to be measured. Determinations based upon 

 responses in form and structure are also distinctly valuable. Because of the 

 longer time involved, they do not permit of such complete control, and their 

 correlation is less exact. In all of these, the error due to individual behavior 

 must be checked out by careful selection of individuals and by using a number 

 sufficiently large to yield a mode and to permit the elimination of those which 

 depart widely. In addition it has proved increasingly desirable to use a 

 battery of two or more species as phytometers, since this increases the number 

 and accuracy of the results quite out of proportion to the extra labor involved. 



The first application of the phytometer method was made by Clements and 

 Weaver (Clements, 1918 :288; 1919) at Pike's Peak in 1918and 1919. The plants 

 used were sunflower, beans, oats, wheat, sweet clover, and raspberry, Rubus 

 strigosus. These were grown in sealed containers, with plants in open pots as 

 checks on the conditions for favorable growth in the former. The normal 

 number of pots for each species was 3 to 5, but this was often reduced by mis- 

 haps. Three series were grown during the summer, the period varying from 

 28 to 45 days. The habitats measured were those of the short-grass associa- 

 tion at 6,000 feet, the half-gravel associes, the gravel-slide associes, and the 

 Pseudotsuga consociation at 8,500 feet, and the Picea engelmanni consocia- 

 tion at 9,000 feet. Stations were visited each week for the purpose of making 

 weighings and of reading the various recording instruments. The responses 

 primarily considered were transpiration and growth, though photosynthesis 

 was measured also. These showed marked differences with reference to alti- 

 tude, degree of shade, and seasonal factors. The relative values were the same 

 for the native Rubus as for the cultivated plants, and the complete results seem 

 to leave no question of the paramount importance of plants for the quantita- 

 tive study of habitats and communities (plate 7). 



The use of several dominants in a phytometer battery amounts almost to 

 employing a plant community as a measure, and suggests the possibility of 

 utilizing portions of actual communities in this way. The simplest way of 

 doing this at present is by means of permanent quadrats which are visited each 

 month or each year and growth actually recorded by height or volume meas- 

 ures or by weight. Since many communities containing both dominants and 

 subdominants, such as Stipa with Amorpha canescens, Psoralea tenuiflora, and 

 Brauneria pallida, occur throughout the area of most climaxes, a series of 

 quadrats containing essentially the same population can be established through 

 a wide range of conditions. Locally, where diverse habitats are found within 

 short distances, as in the case of zones about ponds and of dynamic areas, it is 

 not difficult to transfer soil-blocks of the same community to several different 

 habitats and to follow their behavior in terms of the growth and abundance of 

 the species concerned. Such communities afford the best possible measure of 

 the serai habitats and reactions typical of succession, especially when recip- 

 rocal transfers are made between two contiguous or successive stages. 



