SHELDON: KOEBERLINIA SPINOSA. 99 



becomes almost black on exposure, with thin yellow or nearly 

 white sapwood of twelve to fifteen layers of annual growth. 

 Distribution: Dry gravelly mesas and foothills, valley of the 

 Lower Rio Grande, Texas, westward to southern through 

 northern Mexico." 



The four or five plants seen by Mr. Peace were of various 

 sizes, the largest — the one from which the specimens were 

 taken — being about three feet wide and very compact in 

 growth. The branches were pale green in color, no leaves, the 

 flowers whitish to cream color. This was about three miles 

 southwest of Tucson. 



As far as observed every branch is a thorn, some of them a 

 little blunt at the end and not tipped with the brownish needle- 

 like point. The thorns vary from 1.5 cm. to 7 cm. in length, 

 being from 1 mm. to 4 mm. in diameter at the base. The 

 largest stem obtainable for study was about 4.5 mm. in diam- 

 eter, and was probably five or six years old, possibly more. To 

 all appearances the bark is smooth, but microscopically it is 

 somewhat rough, the roughness due to the dome-shaped pro- 

 jections which occur over the stomatal chambers. The flowers 

 are borne along the sides of very small sharp thorns. They 

 have the appearance of being in a raceme — an umbel-like 

 raceme, some authors call it. The leaves, said to be scalelike, 

 are soon dropped. 



INVESTIGATION. 



The study of the desert plants was taken up in February, 

 1909, in connection with other graduate work at the University 

 of Kansas, and continued until the middle of August of that 

 year. 



The methods employed in pursuance of this study were vari- 

 ous. The chlor-zinc-iodide treatment of sections fresh from the 

 formalin or paraffin, or softened from the dried state, proved on 

 the whole the most satisfactory. 



All the figures, except a few of the flower and some diagram- 

 matic ones, were made by the aid of the camera lucida, the pro- 

 jection lantern or the photographic camera. 



Stem. — The stem as a whole shows some interesting features 

 in cross section, of which the thick epidermis, deeply sunken 

 stomata, the deep palisade tissue, compact wood and small 

 water tubes are not the least striking. 



The tissues might be taken in their order, beginning with 

 the innermost one. It has been observed that no generalization 



