radiation: a general introduction 



87 



four years remains for the present a 

 suggestion only. Clements and Shelford 

 (1939), although they are, in general, fa- 

 vorable to the idea of a correlation between 

 the eleven-year sunspot cycle and biological 

 events, are able to cite few well authenti- 

 cated cases, and they emphasize, rather, 

 cycles that are near or under ten years in 

 length. Elton (1942), following MacLuiich 

 (1937) and his own unpublished data, has 

 definitelv abandoned the suggestion that 

 the rabbit cycle of the Canadian forests cor- 

 responds to the eleven-year sunspot cycle. 

 Even the oft-cited cycles in tree rings of 

 the giant sequoias of California were re- 

 ported by Huntington (1932) to supply 

 "another type of evidence of this same cycle 

 of about ten years." Douglass (1936) re- 

 cords cycles in tree rings of 5.7, 8.5, 10, 14, 

 17, 19 or 20, and 23 years and "certain 



cycles close to 12 years in length." It is 

 perhaps worth noting that the much-dis- 

 cussed eleven-year cycle is not listed in this 

 latest summary. This point seems to trouble 

 Douglass (1936, p. 132), who remarks that 

 "the disturbing feature in all comparisons 

 between solar and terrestrial cycles has been 

 the presence of other cycles on the earth 

 of yery different lengths and only rarely one 

 of 11 years." Because of his hypothesis of 

 a cycle complex, he concludes, however, 

 that "We feel justified in assuming the 

 hypothesis that there is a physical relation- 

 ship between our climatic conditions and 

 the sun." Elton (1942) records his behef 

 that we will eventually be led "back to very 

 curious meteorological and perhaps astro- 

 nomical processes as well as to new rela- 

 tions between climate, physiology, and 

 disease." 



5. RADIATION: A GENERAL INTRODUCTION 



The eflFective environment is holocoenotic; 

 it is a whole composed of many parts as a 

 rope is made of many strands. For the next 

 several chapters holocoenotic aspects will 

 be mainly disregarded, and the approach 

 will be frankly analytical; near the end of 

 the discussion, however, an attempt will be 

 made to bring the strands together again 

 into a unit. For the moment we will focus 

 on one factor or on one set of factors at a 



time. 



RADIATION 



Radiation that reaches the earth from the 

 sun as heat and light has obvious impor- 

 tance for living things. All functional ecol- 

 ogy is closely related, directly or indirectly, 

 to the capture of radiant energy that origi- 

 nates in the sun. Radiations are transmitted 

 in straight lines and are usually thought of 

 as consisting of waves or pulsations which 

 although of different lengths, travel at a 

 velocity of about 3 X 10'" cm. /sec. Some 

 phases of the physics of radiation are most 

 readily explained on the assumption that 

 the radiating units are corpuscles rather 

 than waves. This phase of the matter can 

 be left to the physicists, since ecological as- 

 pects can be stated with approximate 

 accuracy in terms of the wave theory. 



The lengths of the waves, or pulsations, 

 differ tremendously. They extend from the 



long waves of radio, thousands of meters in 

 length, to the x-rays, gamma rays, and 

 cosmic rays only a small fraction of an 

 angstrom unit long (an angstrom unit (A) 

 equals 1 X 10"* cm.). Those of known 

 ecological significance are (a) the infra-red 

 rays that are important for the heat they 

 carry and that range from about 0.1 mm. 

 (100 M') or somewhat longer to 7700 A 

 (1 H = 10,000 A) and are not visible to the 

 human eye. Then (b) comes the narrow 

 octave that we know as light; this extends 

 from 7700 to 3900-4000 A and transmits 

 heat as well as light. The exact limits of 

 visible light yary from person to person and 

 from one species of animal to another. Be- 

 yond these are (c) the ultraviolet rays, 

 which, like those of the infra-red region, 

 are invisible to man. Solar radiation re- 

 ceived at the earth's surface extends from 

 about 135.000 to about 2860 A and lies 

 mainly ^^^thin the wave lengths of 30.000 

 and 3000 A. There is a sharp maximum at 

 4700 A. The earth radiates as well as re- 

 ceives radiations. Coming from a cool body, 

 these lie mainly between 40.000 and 

 500,000 A (4-50 n), with a maximum at 

 95.000 A. 



Water vapor absorbs solar radiation dif- 

 ferentially, with the absorption mainly 

 taking place in wave lengths of 8000 A or 

 longer, a region that lies well beyond the 



