262 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



ular margins and the first three forms of postseasonal growth appar- 

 ently are somewhat more typical of interseasonal growth rather than 

 of intraseasonal. 



The intensity, and perhaps the duration, of the impact of alternating 

 growth-promoting or growth-prohibiting factors determine when and 

 how often trees put down xylem. Insofar as visible results are con- 

 cerned, certain environments probably have one grand and no sec- 

 ondary growth periods a year whereas others have multiple periods in 

 a year. Such secondary periods are well known ; the resultant addi- 

 tional growth ranges from an expanded bud or a single tracheid to an 

 extra tip flush or a sharp, complete, entire growth layer so clean-cut 

 it would be "counted" as an "annual ring." Of course, the sharp, com- 

 plete, entire intra-annuals are not too common even at the lower forest 

 border. These added to the sharp lenses create an error as high as 15 

 percent or more in a straight count of "annual rings" in trees grown 

 under lower forest-border conditions. This is the factor of mul- 

 tiplicity. 



The influence of rainfall, or more properly speaking, the control 

 exerted by fluctuations in soil moisture, is of prime importance to tree- 

 growth studies, and an extensive study of it goes far beyond the scope 

 of the present work. Water, of course, has a critical place in physio- 

 logical processes ; it is fundamental to the hydrostatic system of trees. 

 Some students emphasize the role of water. But if water is a con- 

 trolling factor in certain habitats, one must weigh carefully the fact 

 that decided alternations of wet and dry periods will alternately pro- 

 mote and inhibit growth, no matter in what part of the general grow- 

 ing season they occur. Other students stress the combined influence 

 of many growth factors. Unless an attempt be made to isolate one 

 factor in order to determine its influence on tree growth, much in- 

 formation, both on tree growth and on climate, may remain unknown. 



At the present moment, only such notes on the influence of rainfall 

 will be given as are connected intimately with this study. The apple 

 tree, WAp i, was closely observed throughout 1944. Tip growth had 

 practically ceased by June 11, and a covered terminal bud was set by 

 June 14. This was also true for WCh 3. For the last week of May 

 and the first two of June, the temperatures were high and rainfall was 

 ineffective. Until April 29, the spring had been humid and cold, 

 growth had been severely retarded. Scattered showers of little or no 

 effectiveness marked June and July with the exception of a severe 

 storm and heavy rainfall of June 19, an inch or more of rain falling 

 the night of July 13-14, and a heavy rain the night of July 19-20. 

 Temperatures were high. In any case, a portion of the branches on 



