192 KANSAS UNIVERSITY SCIENCE BULLETIN. 



plain pits, which give the appearance of stone cells (fig:. 21). 

 The xylem occupies about two-fifths of the entire stem 

 (figs. 1, 2 and 11). It is made up of wood parenchyma, 

 wood fibers, fiber tracheids, and tracheal tubes. Secondary 

 thickening of the cell walls has taken place to a large extent. 

 The cells in their growth have become crowded, and there is 

 very little intercellular space to be found, and this decreases 

 as the xylem advances toward the phloem. No distinct rings 

 of growth occur. This seems to be characteristic of many of 

 the desert plants, which may be the result of the short growing 

 season. The cells of the xylem are so similar in appearance in 

 cross section that medullary rays are distinguished with dif- 

 ficulty even in young stems, but they may be seen In tangential 

 and radial longitudinal sections (fig. 11). They are made up 

 of from one to three rows of cells tangentially. According to 

 Solereder, this is a characteristic of the anatomy of the family 

 MalpighiaceEe. As seen in radial longitudinal section, they 

 are thick-walled cells, elongated vertically, and have many 

 plain pits (fig. 19). 



The cells of the xylem give the lignified reaction with c. z. i. 

 and phloroglucin. They are exceedingly hard because of the 

 deposit of silica in the cell wall. It was with diflRculty that 

 xylem tissue could be cut with the razor. After soaking it for 

 ten weeks in hydrofluoric acid and washing the acid out in 

 running water, both longitudinal and cross sections were 

 easily cut with the rotary microtome from material embedded 

 in paraffin. 



The water-conducting system, seen in the stem cross section, 

 is conspicuous. Considering this .system from the stand- 

 point of the percentage of the total area of the stem taken up 

 by tissue devoted to the function of carrying water, it could 

 be considered fairly well developed. The tracheal tubes aver- 

 age from .01 mm. to .052 mm. in diameter. The number of 

 tubes counted in a cross section of a stem of 1.6 mm. in diam- 

 eter was 260, and we find that about 10 per cent of the stem's 

 cross section is occupied by tracheal tubes. Near the pith the 

 spiral tubes are found, while toward the phloem the reticulated 

 tubes are found. These tubes are well supplied with pits, all 

 of which are bordered (figs. 14 and 18). 



Besides the tracheal tubes, many fiber tracheids are also 

 found. These occur scattered through the xylem, intermixed 

 with wood fibers and wood parenchyma. They resemble the 



