FACTOR INDICATORS. 79 



seems to be no question that they vary considerably in their ability to obtain 

 water from the same soil. This is obviously to be explained in part by the 

 fact that the roots are not at the same level, and hence not in the same soil. 

 But there are many cases in which certain species wilt before others, where the 

 roots are interwoven in the same soil. As already mentioned, Dosdall (1919) 

 has found that Equisetum arvense regularly wilts before Helianthus annuus 

 and Phaseolus vulgaris when their roots are at the same depth in uniform soil. 

 This agrees with results obtained in the field at the Alpine Laboratory with 

 uniform gravelly soils, and indicates a considerable difference in the absorbing 

 power of native species. This may be due to striking differences in the rate 

 of transpiration or of the osmotic pressure of the root-hairs, or it may arise 

 from differences in the extent and growth of the roots themselves. As Shull 

 (1916 : 27) has suggested, it would appear less under moderate and uniform 

 conditions, and it seems likewise that it would be less in evidence with crop 

 plants and weeds which grow in fairly uniform root environments. It seems 

 clear that this point must receive further investigation. Meanwhile, it is 

 necessary to recognize that species of the same local group and habitat do 

 wilt at different points, whatever the various causes may be. 



In the endeavor to definitize the significance of water indicators, the primary 

 division into hydrophytes, mesophytes, and xerophytes will still have value. 

 In addition to the subdivision which Warming has already made of them, 

 they will require still further analysis. This will become possible only with 

 more exact study of the controlling factors, and especially of the actual water 

 use. In fact, the precise meaning of any particular indicator will depend 

 wholly upon the latter, and this will involve a readjustment of the relations 

 of the main groups. Meanwhile, a keen appreciation of the need for more 

 exact methods should not be allowed to obscure the fact that indicators of 

 great practical value can still be made available by our present methods of 

 ecological observation and instrumentation. 



Light indicators. — In spite of the fact that small differences in light values 

 are more readily detected by observation than with water-content, the 

 recognition and use of plants as indicators of different light intensities are 

 matters of recent development. The forester has long understood the general 

 importance of light in the forest, and his tables of tolerance are an indirect 

 recognition of indicator values. As long as he was chiefly interested in sil- 

 viculture, however, tolerance was a matter of relative growth in the same or 

 similar situations. The development of silvics as a phase of ecology directed 

 attention more to the factors of the habitat, and led to the use of photometers 

 for measuring light intensity. This has made possible the correlation of tables 

 of tolerance with measured intensities and the use of the dominants con- 

 cerned as direct indicators. Such work has merely been begun, however, and 

 much quantitative study will be required before the general values of tables 

 of tolerance can be made exact. Measurements of light intensity have been 

 largely confined to forests, but it is clear that light values have considerable 

 importance in other communities as well. This is especially true in wood- 

 land, scrub, and savannah, but it holds also for grassland, particularly the 

 tall-grass prairies. 



Two facts must be taken into account in correlating light indicators with 

 measures of light intensity. One of these is the effect of variations in the 



