SIZE VARIATION IN TRACHEARY CELLS. 197 



length of tracheary elements, there are indications of other variations 

 in cell length which seem to be induced by different factors. 



In the Coniferae — among plants of comparable ages — the smaller, 

 sloAver growing forms, e. g. certain Taxoceae and Cupresseae, tend to 

 have shorter elements than larger, more rapidly growing forms. 

 Furthermore, within the same species, dwarfed or depauperate plants 

 tend to be composed of shorter tracheary elements than normal indi- 

 viduals. Similar tendencies occur among the dicotyledons. It is 

 important to note in this connection, that the tracheary elements 

 formed about the junctions of stems and roots, branches, or leaf traces, 

 wound tissue, compression wood, and similar tissue in which growth 

 adjustments are taking place, tend to be considerably shorter than 

 those which occur in normal internodal or straight grained wood, 

 Figs. 8, 4, and 5, Tables IV and V. Of course, the abundance and 

 distribution of this short celled tracheary tissue varies considerably 

 in plants grown in different environments and in plants of different 

 growth habits. Thus, tall forest trees whose stems are devoid of 

 branches over a considerable length of their stem, appear to have 

 longer tracheary cells than non-gregarious trees whose stems are 

 clothed nearly to the level of the ground with large branches. Simi- 

 larly, gnarled or twisted trees, such as occur commonly in alpine 

 regions and other windswept or exposed situations, have shorter 

 tracheids than erect plants of more sheltered habitats. 



In so far as the length of the tracheary elements of the secondary 

 xylem of vascular plants in general is concerned, there is no absolute 

 correlation between body size and cell size. The tracheary elements 

 in large dicotyledons may be considerably smaller than those that 

 occur in comparatively small gymnosperms or vascular cryptogams. 

 Similarly a gigantic sequoia may have shorter cells than a small cycad. 

 Although the secondary tracheary elements in the Coniferae and 

 Dicotyledoneae increase in size as the plants become larger, this phe- 

 nomenon lasts for only a comparatively limited number of years, after 

 which the size is subject to comparatively slight fluctuations. 



It is to be emphasized in conclusion that the primary object of an 

 extensive reconnaissance survey of this character should be to blaze 

 the way for subsequent and more intensive investigations. Having 

 blocked out certain of the more important size variations that occur 

 in the tracheary cells of the secondary xylem of vascular plants, it is 

 essential to isolate and study the factors which regulate or control these 

 phenomena. Of course, the length of the tracheary elements of the 

 secondary xylem is dependent upon (1) the length of the cambial 



