440 THE INVESTIGATION OF SUCCESSION. 



measuring light intensity have been based upon the use of photographic paper. 

 Theoretically this is unsatisfactory, because only the blue-violet part of the 

 ray is measured. Practically, however, the use of such photometers for more 

 than 15 years has furnished convincing evidence that it is a very satisfactory 

 method of measuring the effect of light in the structure and development of 

 communities and the adaptation of species. In the endeavor to organize the 

 whole field of light instrumentation, the writer has designed and used with 

 steadily increasing eflaciency the following series of photometers: (1) simple 

 photometer; (2) stop-watch photometer; (3) water photometer; (4) selagraph, 

 or recording photometer; (5) speetro-photometer. The construction and 

 operation of these are described in detail in a forthcoming paper. In addition, 

 a further effort is being made to develop a method by which standardized 

 plants are employed for determining the amount of photosynthate in different 

 serai stages. 



GROWTH METHODS. 



Bing-coimts. — In determining the successional relations, and especially 

 the sequence of woody dominants and subdominants, determinations of the 

 respective ages by counts of the annual rings is of the first importance. This 

 is especially true of supposed cases of degeneration of forest or its conversion 

 into scrub, heath, or grassland. There is no substitute for this method, 

 except the all but impossible one of tracing the course of development through- 

 out, which would require more than a life-time. It is for this reason that all 

 reported cases of natural degeneration or conversion have been called in ques- 

 tion, as well as many of those where the operation of artificial factors is slow. 

 In none of these have the exact methods of ring-counts and quadrats been 

 employed, and in consequence the conclusions reached can only be regarded as 

 working hypotheses. In the more minute studies of sequences and of dates 

 it has been fotmd possible to determine the ages of perennial herbs, by the 

 rings as well as by the joints of their rhizomes or other imderground parts. 

 This is of particular value in the study of colonization after fire or other 

 denuding forces. Ring-counts can be used to the greatest advantage in ascer- 

 taining the relations of dominants in mictia and in ecotones, but they are also 

 ) indispensable in determining the serai significance of relicts. In fact, the 

 recognition of relicts often depends wholly upon the determination of respec- 

 tive ages. In reproduction, especially under competition, and particularly 

 where forest or scrub is in contact with grassland, the ages of the invading 

 trees or shrubs at various distances from their community is indispensable to 

 a knowledge of the present success and the future outcome of the invasion. 



A detailed accoimt of methods of counting rings seems unnecessary because 

 of the general simplicity of the problem. Certain precautions are necessary, 

 however, as well as great care in the actual process of counting the rings 

 (Clements, 1907, 1910; Douglass, 1909, 1913, 1914; Huntington, 1914). 

 Stump-counts are desirable as a rule, but in many cases the increment borers 

 of foresters can be used to advantage. Fortunately, liunbering and clearing 

 usually furnish the necessary stmnps, though the intensive study of succession 

 over many areas can only be carried on by the constant use of ax and saw. 



