188 KANSAS UNIVERSITY SCIENCE BULLETIN. 



wholly lacking. This deficiency, however, is in a measure com- 

 pensated by the modification of other tissues, the walls of the 

 wood parenchyma, cortex, pith and medullary-ray cells being 

 remarkably thickened. The walls of the cortex and pith cells 

 are, on the average, 0.03 mm. thick. In figures 10 and 11 are 

 shown a few cortex cells as seen in cross and longitudinal 

 sections, respectively. In figures 12 and 13 are shown a few 

 pith cells as seen in cross and longitudinal sections, respec- 

 tively. As shown in figure 14, the tr.icheids have thick, ligni- 

 fied walls and constitute the smaller portion of the xylem area ; 

 these, together with the wood parenchyma cells, tend to 

 strengthen the plant. A longitudinal section was mounted in 

 a drop of concentrated solution of chromic acid ; in a few min- 

 utes everything had disappeared but the tracheids. The 

 chromic acid dissolved the wood parenchyma cells, leaving the 

 tracheids isolated as shown in figure 15. The average length 

 of a tracheid is 0.243 mm. ; the average width, 0.016. mm. 



Tracheal Tissues. — In the stem the conduction of water is 

 provided for by tracheids varying in length, and in longi- 

 tudinal section having the appearance of typical tracheal 

 tubes. When, however, a longitudinal section was macerated 

 under a cover glass in chromic acid, the tracheids were isolated 

 from the other tissues and stood out prominently as spiral and 

 scalariform tracheids. By computing the number of tracheids 

 in one xylem wedge and multiplying by the number of such 

 wedges, where the cross section of the stem is 19.635 sq. mm. 

 in area, there were found to be 1313 tracheids. The average 

 diameter of these stem tracheids is 0.016 mm. The total 

 water-conducting area of the stem, cross section, is 0.266 

 sq. mm., which is 1.3 per cent of the whole stem. Of the xylem 

 portion of the stem, 6.6 per cent is given up to tracheids. 



When compared with the water-conducting tissue of several 

 mesophytic herbaceous plants, that of Toivnsendia seems quite 

 small. In Saponaria officinalis 22.7 per cent of the whole stem 

 is devoted to the conduction of water; of the xylem portion 

 29 per cent, the average diameter of a tracheal tube being 

 0.026 mm. Thus it is seen that 17 times as much of the stem 

 of <S. officinalis as in Toivnsendia is devoted to the conduction 

 of water. 



In Verbena stricta there was devoted to the conduction of 

 water 9 per cent of the stem cross section, where the area was 



