SIZE VAUIATIOX IN THACHKAUY CKLLS. 1 !).") 



and Dicotyledoneae — frequently appear to bo relatively long and of 

 nearly the same length, if not actually longer, than the outermost ele- 

 ments of the primary wood, (Iraph 1, Fig. 1. This is in marked con- 

 trast to the conditions which occur in the rest of the gymnosperms and 

 woody dicotyledons, in which the primary tracheary elements average 

 as a rule considerably longer than the innermost elements of the 

 secondary xylem, Ficjs. 1 and 2. In other words, the most striking 

 differences in length between the tracheary elements of the higher and 

 lower gymnosperms occur commonly in the innermost cells of the 

 secondary xylem, Figs. 1 and 2. Furthermore, whereas, in the older 

 gynniosperms and vascular cryptogams, the secondary xylem of small 

 plants may be composed of relatively long tracheids, the smaller 

 growth forms of the Coniferae tend to have comparatively short 

 tracheids even in the outermost portions of the secondary wood. 



Summary. 



A comparative study of the secondary xylem of vascular plants 

 reveals a number of interesting facts in regard to the length of the 

 secondary trachear}' elements in vascular cryptogams, gymnosperms 

 and angiosperms. The tracheary elements of the secondary xylem 

 in the vascular cryptogams tended to be very long, whereas those 

 which occur in the dicotyledons — with the notable exception of the 

 vesselless Trochodendroceae and Magnoliaceae — are comparatively 

 short. The gymnosperms appear to occupy an intermediate position 

 between these extremes; the Cordaitales, Bennettitales and Cycadales 

 resembling the vascular cryptogams, and the Gnetales — supposed 

 gymnosperms with vessels simulating the angiosperms. 



The reduction in the size (length) of the tracheary elements of the 

 higher plants appears to have proceeded along certain general and 

 more or less distinct lines. 



(1) In all the dicotyledons and gymnosperms with the exception 

 of the Cordaitales, Bennettitales and Cycadales, the first formed tra- 

 cheary cells of the secondary xylem are relatively small, and in all of 

 the material examined by the w riters are considerably shorter than the 

 adjoining elements of the primary xylem and subsequently formed 

 tracheary cells of the secondary xylem, Figs. 1 and 2.^ 



6 In dicotyledons having very highly differentiated types of vessels, the short 

 vessel-segments may be of nearly uniform size in succeeding growth layers of 

 the secondary xylem. 



