RESPONSE OF FRUIT PLANTS TO CONDITIONS OF SOIL 



69 



paratively short period of rapid vegetative growth. Figure 9 presents 

 graphically the results of his series of observations upon yellow pine 

 seedlings near Flagstaff, Arizona. 



In commenting upon the data presented in this figure he says: "Contrary 

 to what might be expected, there is no apparent relation between height growth 

 and annual precipitation, summer precipitation or winter precipitation, m fact, 

 the growth from year to year often varies inversely with the precipitation for 

 any of these periods. When it is considered that of the total annual precipi- 

 tation at Fort Valley, the mean amounting to about 23 inches, approximately 40 

 per cent, comes during the winter months (December to March), 30 per cent, 

 during July and August, and less than 10 per cent, during the spring months 

 (April and May), the foregoing statements are startling. In order to clarify the 

 problem, it is necessary to analyze the growth habits of Western yellow pine as 

 well as the cUmatic and soil conditions under which it grows in this locality. 



1915 



1916 1917 



1909 1910 1911 1912 1913 



Year 



Fig. 9. — Seasonal precipitation and annual height growth of western yellow pine 

 saplings from 1909 to 1917. a, Annual precipitation; h. Winter precipitation (December- 

 March preceding the corresponding year's growth) ; c, Summer (July-August) precipita- 

 tion; d, Annual height growth; e, Spring (April-May) precipitation. {After Pearson y^"^) 



The terminal shoots begin to elongate about the middle of May, and by July 1 

 they have practically completed their growth. Thus it appears that the entire 

 height growth occurs during the period of lowest precipitation of the year. From 

 the middle of May to the middle of July the rainfall is normally less than one half 

 inch, and comes in such small showers as to be of no benefit to deep-rooted plants. 

 It is evident,therefore,that the moisture utihzed in making this growth is drawn 

 almost entirely from a stored supply. It is also evident that the midsummer 

 rainfall, since it does not begin until July, when height growth has practically 

 ceased, is of httle or no consequence, as far as the current year's height growth is 

 concerned. The water storage which makes growth possible is mainly the 

 result of the preceding winter's precipitation; but it is the supplementary sup- 

 ply in April and May which determines whether the growth is to be above or 



