278 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



mean a wet year and, where it does correlate closely with seasonal 

 rainfall, such correlation does not necessarily encompass the entire 

 year. It is, however, during the period when growth occurs that a 

 combination of factors closer to the over-all optimum than to the 

 average, creates good growing conditions and therefore much tree 

 growth. 



Postseasonal growth, no matter when it occurs, whether soon after 

 the main growing season or during the following winter, poses a dif- 

 ficult problem for the student who correlates tree growth with rainfall 

 at the lower forest border. 



MULTIPLICITY AND CYCLE INTERPRETATIONS 



Even if cycles were not intriguing, it would be proper and it would 

 be a duty to hunt them out wherever possible because any repeating 

 pattern in an environmental factor can have high significance for the 

 life forms involved. The search for cycles should, nonetheless, be 

 grounded upon reasonable theory, sound principles, and sharp ob- 

 servation. Superfluous as the words may be to some, it still seems 

 necessary to say that a cycle — its reality, its validity, its length, its 

 amplitude, and its continuity — is no better than the data which ap- 

 parently yield the cycle. This subject of cycles concerns the present 

 work in several ways. 



If a plotted graph of growth-layer thicknesses shows cyclic varia- 

 tions, the natural assumption that they are caused by rainfall fluctua- 

 tions follows. A reasonable question may then be asked : If cycles in 

 growth-layer thicknesses are held to mirror cycles in rainfall, then 

 should not the rainfall itself show the same cycles in simpler, purer 

 form? Also, certain compHcations intervene between rainfall and its 

 entrance into the ground, between its entrance and its conversion into 

 soil moisture, between soil moisture and the hydrostatic system of the 

 plant, and between the hydrostatic system and the physiologic proc- 

 esses which ultimately result in the production of xylem and phloem. 



Unless growth-layer thicknesses are actually influenced by the rain- 

 fall of the entire year, they cannot be expected to reveal the cycles in 

 annual rainfall. 



Tree growth commonly has been found to compare more or less 

 roughly with the rainfall of a period less than a year in length (Glock, 

 1941, pp. 687-689). Therefore, cycles in tree growth could only re- 

 flect cycles in period rainfall. Is it theoretically possible to have cycles 

 in, say, spring rainfall? A study of this possibility would seem pre- 

 requisite to a study of cycles in tree growth. 



