282 KANSAS UNIVERSITY SCIENCE BULLETIN. 



The phloem in the leaf vascular system is made up of undi- 

 vided mother cells of sieve tubes, and cambiform cells. These 

 have much the same appearance in size and shape, both vary- 

 ing- from .007.5 to .0087 mm. in cross diameter, and in length 

 from .009 to .0182 mm. The average thickness of the cell wall 

 is .00012 mm. 



In the case of a leaf infected by a fungal growth there were 

 found cells lying just without the phloem, which showed a 

 peculiar structure. They were sclerenchymatous in appear- 

 ance, and when treated with chloriodide of zinc they were seen 

 to have a secondary core of cellulose, these lying in a primary 

 lignose matrix. (See fig. 45.) 



THE STEM. 



The stem is herbaceous, with fine pubescence at the growing 

 apices, tending to become woody at from one to two inches 

 back from the apex, and as it approaches the base of the shrub 

 the stem covering becomes ven^ rough, dark, and deeply 

 seamed. The woody part of the plant is highly resistant to 

 sectioning, hydrofluoric acid having little or no effect upon 

 portions of the wood, even after continuous application for 

 some four months. 



The most interesting features of the stem are those seen in 

 cross section, the fluctuation in extent and position of bast and 

 vascular tissue being the most striking. This variability is 

 shown in the series of drawings, figures 1, 2, and 3. The sec- 

 tions used for these representations were taken from a series 

 cut in paraffin, young parts of the stem being taken a few 

 centimeters back from the growing apex. In describing each 

 tissue in detail there will be a definite order followed, begin- 

 ning with theinnermost one. 



The Pith. 



The pith occupies a somewhat pentagonal area ; the indi- 

 vidual cells are also pentagonal to circular in shape, and in 

 the very young stem, within 3 mm. of the growing apex, have 

 a cellulose cell wall approximately .0014 mm. in thickness. 

 (Fig. 5.) At a distance from 1 to 3 cm. back from the grow- 

 ing apex, however, the walls take on a heavy secondary thick- 

 ening and become highly lignified. The secondary thickening 

 begins with the cells at the center of the pith area, and grad- 

 ually extends outward until all the walls of the pith have 

 taken on this thickening and lignification. In this latter con- 



