668 Forestry Quarterly 



The resiilts secured by the writers, in their study of fibers and 

 vessel-segments in hickory, are, accordingly, of interest as afford- 

 ing a possible clue to the variations of these xylem elements in 

 the development of arborescent Dicotyledons, and the com- 

 parative behavior of representatives of the Angiosperms and 

 Gymnosperms. As is shown graphically in figure 1, fiber length, 

 in Carya ovata, increases rapidly for a number of years (20-25) , 

 and does not become constant in succeeding years. It is subject 

 to regular cyclic variations such as occur in the tracheid length of 

 all conifers investigated, except the material of Pinus sylvestris 

 examined by Sanio. The increase in length is, however, less 

 rapid, the maximum length attained is considerably shorter, and 

 the cyclic variations are less marked than in conifers. The 

 length of the vessel-segments increases rapidly for a few years, but 

 appears to remain nearly constant during the later stages of the 

 tree's development.'' The diameter of the vessels, on the contrary, 

 increases considerably during this period. 



A survey of all the evidence at hand indicates that in the 

 development of the stem of Coniferae and many large perennial 

 Dicotyledons there is a period, in the early stages of the plant's 

 life history, during which the woody elements increase in size com- 

 paratively rapidly. The duration of this period and the rate 

 of increase during the period vary more or less in different groups 

 of plants, in different specimens of the same species, and at differ- 

 ent heights in the stem of a single individual. Furthermore, dif- 

 ferent types of xylem elements, e. g., tracheids, libriform fibers, 

 and vessel-segments, behave very differently. 



One of the writers,^ working in collaboration with Mr. W. W. 

 Tupper, has made a study of the length of the xylem elements in 

 the secondary wood of a large number of Gymnosperms and 

 Angiosperms. This investigation has shown that, in comparable 

 mature material, the tracheids of the former group of plants are, 

 on the average, more than twice as long as the tracheids, fiber- 

 tracheids, libriform-fibers, and vessel-segments of the Dicotyledons. 

 The tentative asstmiption may be made, accordingly, that the 

 curves in figure 1 are, in a general way, indicative of the differ- 



^ The fluctuations in the curve are but sHghtly greater than the probable 

 deviations of the means of twenty measurements. 



* Tupper, W. W., and Bailey, I. W. Some observations upon the secondary 

 xylems of Gymnosperms and Angiosperms. Science, XLIII, No. 115, p. 323, 

 March, 1916. 



