288 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I4O 



Evidence thus suggests that cambial activity, differentiation, and 

 maturation begin locally on the aerial plant body, spread out, more or 

 less, and cease. Incipient flushes of growth, in other words, give rise 

 to partial growth layers and, unless prohibited, will ultimately spread 

 into an entire sheath of xylem covering the aerial plant body. This 

 sheath will continue to increase in thickness until growth factors cease 

 to be favorable. 



MULTIPLICITY 



The subject of multiplicity adds the time factor, with emphasis, to 

 the types and classes of growth layers included under the subject of 

 Classification. Growth may be localized not only in place but also in 

 time. 



Investigation reveals that growth may cease at any time after it 

 begins and may be reinitiated at almost any time after it ceases ; that 

 the amount of xylem may be limited to a few cells only or may include 

 a layer over the entire aerial plant body ; that growth rhythms are not 

 necessarily geared to an annual cycle ; that one or several flushes may 

 be recorded by xylem in one year; that the growth layers resulting 

 from intra-annual flushes may, and commonly do, possess outer bor- 

 ders indistinguishable from the borders terminating the annual incre- 

 ment; and that a partial growth layer does not represent an annual 

 increment except for one percent or less of the several thousand par- 

 tial growth layers that have been dated with exactness. 



The studies included under Multiplicity emphasize the fact that 

 growth flushes, and the intervals between them, are distinctly variable 

 in length. 



A merging of the results obtained from studies of Classification and 

 Multiplicity indicates the place, the time, and the intensity of a growth 

 flush. The visible result of a growth flush is one of the many varieties 

 of growth layers. Thus, we may define a growth layer as follows : It 

 includes the xylem laid down by the cambium during a single period 

 of activity regardless of entirety around the circuit, of completeness 

 radially, of location within the plant body, or regardless of the partic- 

 ular time of the activity. 



As a matter of fact, growth processes are predominantly and in- 

 tricately rhythmic — they may leave traces of themselves in the xylem 

 which are, so to speak, invisible as well as visible. 



IX. CONCLUSIONS 



The discovery and use of absolute dating offer a unique opportunity 

 to trace in some detail the history of growth flushes, and to determine 



