RESPONSE OF FRUIT PLANTS TO CONDITIONS OF SOIL 



67 



should be watered. . . , With varying times of application of irrigation 

 water the season of most rapid twig growth is during the season of 

 watering."'^ Barss,^ reporting upon the results of some pot irrigation 

 experiments with pears, states: "The most noticeable variation in 

 response to the application of different amounts of water, was found in the 

 development of the new wood. All the lots started vegetative growth at 

 about the same time . . . but terminal bud formation took place early 

 on the poorly watered trees and much later on the trees of the other lots. 

 Furthermore there were great differences in the rate of wood growth in 

 these different lots while they were actually growing. . . . The spurs 

 on the better-watered trees were larger and more vigorous. . . . From 

 leaf samples, collected and weighed in order to bring out any existing 

 differences in weight, it is apparent that, on the average, the leaves 

 in the lots receiving most water were far heavier than those in the lots 

 receiving less water." He also found the leaves on the trees receiving the 

 smallest water supply were variable in both size and shape, their petioles 

 were slender, their lower surfaces were markedly pubescent and their 

 color was dark green. Callus tissue formed much more frequently on the 

 well watered trees. 



An7iual Rings and Trunk Circumference. — The results of study on the 

 relation between tree growth and total yearly rainfall in Arizona are 





1870 1880 1890 1900 l^iQ 



Fig. 8. — Actual rainfall compared with rainfall calculated from growth of trees, 

 Arizona. Solid line equals calculated rainfall. Broken line equals observed rainfall. 

 {After Douglass J^) 



interesting in this connection. Under the comparatively arid conditions 

 of that region the correlation between the two was found to be so close 

 that with a knowledge of the total rainfall of any one year the average 

 increase in diameter of trees could be estimated with an average accuracy 

 of 82 per cent.; conversely, knowing the average diameter increment 

 of a small group of forest trees for any one year it was possible to estimate 

 with equal accuracy the total precipitation of that year. Figure 8 shows 

 graphically the closeness of this correlation. Huntington^^ employed 

 this method of estimating annual rainfall for the study of climatic varia- 

 tions during the last 1,000 years, obtaining growth records from the giant 

 Sequoias of California. Hartig,^^ however, found that in parts of 

 Germany where low moisture content of the soil apparently is not the 

 limiting factor to growth, the beech makes a smaller annual ring during 



