X74 KANSAS UNIVERSITY SCIENCE BULLETIN. 



present, but with the cell walls always much thicker. Even 

 here there are no actual perforations. 



Plants often adopt the device of obliquely placed end plates, 

 possibly for the purpose of furnishing greater surface for the 

 pits. This is noticeably true in many instances, especially in 

 the Salicacese. As a result it seems necessary to strengthen 

 these walls, and cross banding with thickenings in the walls 

 are seen in figures 2, 8 and 9. The best illustrations (figs. 8 

 and 9) are from a root of Populus deltoides, taken about eight 

 feet below the surface of the ground. Here there is a much 

 increased sieve-plate surface, with strengthening bands at 

 regular intervals, as seen from two directions. 



Problem II. — Are sieve tubes always present in the phloem? 



As to this problem, my data, many of which were obtained 

 only after long search, are quite positive. Plants differ greatly 

 in arrangement, location, number, size, staining qualities, etc., 

 of their sieve tubes in the various specimens. However, one 

 can be reasonably sure of their presence by noting the phloem 

 in cross section. The companion cells and the sieve tubes ad- 

 joining form units in outline, which, together with the proto- 

 plast of the companion cell and the slime body of the sieve 

 tube, assist in the discovery (fig. 6). Sieve plates are not so 

 easily found, unless it may be in those plants where sieve 

 tubes are numerous, or where one happens to get a section 

 through an unusually favorable place. For still better evi- 

 dence the slime body is good, but the perforated sieve plate is 

 best. I was usually not satisfied until all of these requirements 

 were met, and the drawings were then made. 



Perforations were not found in the end plates in Pirnis 

 laiicio, Sagittaria, and Asparagus. They were quite numer- 

 ous, although very small, in Opuntia. In Myriophyllum, a 

 hydrophyte, they were evident, as in Cuscuta, a parasite. In 

 this last were found cells of large dimensions, apparently 

 serving especially as conducting cells. One was 0.1 mm. In 

 tangential diameter by 1.62 mm. in axial length. We could not 

 tell how much longer, because of the limits of the section. 

 Sieve tubes are also present in Tillandsia, an epiphite. Here 

 there is a cylinder of sclerenchyma inclosing the vascular bun- 

 dles, and probably permitting of but little lateral transfer of 

 foods. 



Data concerning the sieve tubes are contained in tables I-IV. 



