Ramaley: anatomy of onagraceae. 687 



elements, the cells being placed in rather definite radiating 

 rows. In those species where intra-xylar phloem islands occur 

 this regularity is, however, broken in the parts just exterior to 

 the islands. At these points the cells may be quite irregularly 

 placed. 



The vessels of the secondary xylem are usually fritted, though 

 occasionally reticulated. They are often in groups of three, 

 four or five, extended in a radial direction. In cross section 

 the vessels appear irregularly polygonal or elliptical; the long 

 axis, sometimes placed in a direction parallel to the circumfer- 

 ence of the section, at other times at right angles to it. In a 

 few cases it is somewhat oblique. The partially absorbed trans- 

 verse septa are readily seen in longitudinal section. The indi- 

 vidual elements are seen to be, as a rule, about three times as 

 long as thick. The largest vessels noted in the secondary wood 

 were lOOmik. in diameter (Onagra biennis, Oenothera sinuata). 

 Of the last named species a section 2mm. in diameter showed 

 no vessels over37mik., while one 4mm. in diameter from another 

 plant has vessels of the size above stated. As a rule, species 

 with slender stems have smaller vessels than those with robust 

 stems. There is considerable variety in the number of vessels 

 seen in different cross sections. Apparently the proportion of 

 vessels to wood fibres is by no means constant. 



The wood fibres are elongated fusiform elements, sharp - 

 pointed at the ends. No branched fibres were observed. The 

 walls are often of considerable thickness. In size the fibres 

 average, perhaps, 200mik. in length and 10 or 15mik. in thickness 

 at their widest part. In transverse section they appear more 

 or less hexagonal in outline, fitting into each other without in- 

 tercellular spaces. Occasionally some of the fibres are more or 

 less triangular in cross section (Oenothera humifusa), or small, 

 somewhat diamond-shaped cells may be found between those of 

 hexagonal form (Oenothera rhombipetala). The fibres seen in 

 cross section are commonly elongated in a tangential direction, 

 but in some sections they are found to be at times radially 

 elongated (Galpinsia hartwegii). The middle lamella is. in 

 nearly all cases, distinctly discernible. It shows thickenings 

 at the angles. 



Medullary rays. — The medullary rays extending from the 

 pith to the phloem are very numerous. There are also many 

 secondary rays which take their origin at some distance from 

 the pith. Some of the rays are traceable the whole distance 

 through the phloem to the pericycle. More often, however, 



