Il6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I40 



tervals during any one growing season. Those activities appear to 

 possess not only fluctuations of high amplitude and long wavelength, 

 giving the ordinary growth layers, but also fluctuations of slight 

 amplitude and short wavelength, giving what may be called crypto- 

 growth layers. The short-wave cycle is no doubt superimposed upon 

 the long-wave cycle. 



(3) The third situation has to do especially with transitions in a 

 radial direction. Typically, a growth layer is thought of as being 

 complete if the ratio of lightwood to densewood is rather high in favor 

 of the lightwood. A decrease in width of one commonly accompanies 

 a decrease in the other so that if the lightwood consists of few cells 

 radially, say three or four, the densewood may be reduced to one cell 

 only. Nevertheless, the typical situation has striking exceptions, the 

 one radial, the other tangential, insofar as the lightwood is concerned. 

 It may be reduced in amount radially until the densewood comprises a 

 very high percentage of the total growth layer. Or, the lightwood may 

 be reduced tangentially until it disappears entirely and the densewood 

 of that growth layer then lies sheer against the densewood of the next 

 inner growth layer. This results in a lens of lightwood which under 

 low magnification appears to be a complete and bona fide lens. Such 

 an error in recognition would lead to an entirely erroneous interpre- 

 tation of the regional extent of cambial activity and of the amount of 

 xylem formed during the particular growth flush. 



Certain particulars of the subject in hand should be mentioned in 

 more detail. The amounts and proportions of lightwood and dense- 

 wood, for instance, vary radially and longitudinally in absolute and 

 relative fashion. Apparently the densewood varies to a much greater 

 extent than does the lightwood. In the loblolly pine. Con T 2-o-base a, 

 the densewood of 1943 varies in thickness, whereas the lightwood re- 

 mains constant around the circuit. Arizona cypress is prone to great 

 variations. In TTC 36-7-a, for instance, the densewood of 1938 

 ranges from a thick band constituting a high percentage of the com- 

 plete growth layer to a very thin, weak band constituting a slight per- 

 centage of it. These variations may exist in one growth layer on one 

 section, or they may be on different branches. On a single section, the 

 densewood may be but one cell thick on one radius and may increase 

 in thickness tangentially to many cells far exceeding the lightwood. A 

 growth layer toward the extreme may contain only one lightwood and 

 one densewood cell, or the lightwood cell may fail, leaving a growth 

 layer represented by one row of densewood cells. 



The maple, TTM i, gives a very slight hint that there may be a 

 relationship between amount of tip growth and amount of lignification, 



