688 MINNESOTA BOTANICAL STUDIES. 



the rays cannot be at all easily distinguished in the phloem. 

 The separate elements of the rays are usually rectangular in 

 outline. In cross section they appear elongated in a radial 

 direction. In radial longitudinal section the cells are seen to 

 be vertically elongated. 



All these cells, so far as observed, have pitted walls. Some 

 show marked reticulations and irregular thickenings. An in- 

 considerable amount of lignification is in some cases to be ob- 

 served. Starch is occasionally present in the cell cavities. 

 The starch grains show no peculiarities in form. They are 

 circular in outline; no striations were observed. 



In size the medullary ray cells show some variation. In 

 Oenothera humif usa, Kneiffia linearis and Kneiffia linifolia the 

 average diameters in cross section are generally 7 to 12mik. In 

 some of the larger species the cells have diameters of 10 and 

 20mik. In respect to size, however, there is to be noted the fact 

 that individual variations in plants of the same species may 

 often be very great. 



Pith. — The pith is composed of parenchymatous elements. 

 That portion which is adjacent to the vascular bundles is com- 

 monly much smaller celled than the central part. Its cells are 

 often somewhat flattened and crushed by the growth of the 

 medullary phloem, which appears to be present to a greater or 

 less extent in all the species examined. 



The cells of the central portion of the pith are very large, 

 frequently attaining a diameter of from 50 to lOOmik. Starch 

 may be present throughout. There are sometimes large inter- 

 cellular spaces. Commonly the cells present no peculiarities 

 in structure. In some species occasional thick-walled cells are 

 found which take a liguin stain. These cells occur singly or 

 in small groups. They are especially abundant in Kneiffia 

 glauca, Oenothera humifusa and Kneiffia linifolia. In each 

 case the cell wall shows, in stained sections, distinct lines of 

 stratification. Crystal sacs are in many cases abundant. Seen 

 in cross section they are nearly always circular or elliptical in 

 outline and may be smaller than the ordinary parenchymatous 

 cells, though, as a rule, they are larger. They are closely 

 packed with raphides of calcium oxalate. These crystals are 

 in no way different from those of the phloem and cortex. 

 Crystal sacs may occur in the central portion of the pith; 

 more often, however, they are placed not far from the medul- 

 lary phloem. 



