70 BASES AND CRITERIA. 



tions of the same habitat. In an area which is uniform physically, individuals 

 frequently show striking variations due to competition. These four relations 

 sum up the indicator values of growth-forms as they occur in nature and hence 

 serve as the basis of all correlations. While they are well-known, little 

 quantitative work has yet been done with them. This has been due to the 

 time necessary to organize quantitative studies and methods out-of-doors and 

 to focus these upon growth as the most basic of visible responses. Pearson 

 (1918) has made measurements of the annual growth in height of yellow-pine 

 seedlings for a period of six years and has found a close correlation with 

 spring rainfall. Sarvis (1919) has clipped and weighed the growth on perma- 

 nent grass quadrats at intervals of ten days and has made a general correla- 

 tion with seasonal factors. Since species vary greatly in rate and amount of 

 growth, it is desirable to select those most responsive to the habitat. 



It is impossible to say as yet what type of growth is most readily correlated 

 with seasonal variations or habitat differences. Theoretically, it seems that 

 total growth as indicated by the dry weight of mature plants would furnish 

 the best correlation (cf. Pearson, 1918; Frothingham, 1919; Sarvis, 1919). 

 Actually, however, vegetative growth and reproductive growth make different 

 demands, and are often antagonistic to each other. This is true to a large 

 degree of the height-growth and width-growth of woody plants. The determi- 

 nation of dry weight is a practical impossibility for trees except when young, 

 and the indicator correlation must be with growth directly. At present it 

 is only possible to say that for the first 100 to 150 years height-growth offers the 

 better correlation, and after this period growth in diameter reflects conditions 

 more accurately. Mitchell (1918 : 23) has shown in the case of incense cedar 

 (Libocedrus decurrens) that the mean height-growth for the first 100 years was 

 65 feet, for the second century 28 feet, for the third 12 feet, and for the fourth 

 6 feet. The width-growth was 13 inches, 14 inches, 9 inches, and 5 inches for 

 the same periods. Thus practically 60 per cent of the height-growth was 

 made in the first century, and but 31 per cent of the width-growth, while the 

 height-growth of the fourth century was but 5 per cent in contrast to a width- 

 growth of 12 per cent. The correlation of reproductive growth and especially 

 of seed-production with seasonal or habitat conditions is known only to the 

 extent that it tends to rise with less favorable conditions as to water up to a 

 certain point, as shown by alpine and arid regions. For most woody plants 

 it is little or none in youth, and it increases steadily up to maturity. In the 

 case of crop plants, it seems clear that the correlation with dry weight offers a 

 satisfactory basis for comparison, though even here greater accuracy can be 

 expected from the separate correlation of vegetative and reproductive growth 

 with the controlling factors in the two periods. 



Standard plants for growth correlations. — Because of the control possible as 

 well as the opportunity for measuring functional responses, standard plants 

 offer much the best method of establishing growth correlations. The value 

 of the method increases as the standard plant approaches the one to be 

 indicated in character, and reaches a maximum when the latter is itself 

 employed as a standard, as in the use of yellow pine, Douglas fir, etc., in forest 

 investigations. The employment of phytometers in this form is the most 

 basic of all quantitative methods and is destined to play the paramount r61e 

 in all exact studies of communities and habitats in the future. 



