686 MINNESOTA BOTANICAL STUDIES. 



tend five to six cells in a radial direction and twenty or more in 

 the direction parallel to the surface of the stem. In old por- 

 tions of the stem of Oenothera sinuata the islands present a 

 peculiarity in their arrangement. They are not disposed in in- 

 terrupted circles as they are in the various other species, but 

 appear to be scattered at random. In Onagra biennis the intra- 

 xylar phloem islands are well developed in old parts. Some of 

 these islands show in cross section but four or five cells, and 

 many but seven or eight; still others consist of very consider- 

 able masses of tissue. The cells of this intra-xylar phloem in 

 cross section are commonly small, not differing greatly in size 

 from the smaller wood fibres. The islands first make their ap- 

 pearance about 0.5mm. from the pith. There is, however, con- 

 siderable variation in this respect. 



Cambium. — The cells of this tissue can, as a rule, only with 

 difficulty, be recognized. They present no peculiarities in form 

 and structure. In cross section the cambium, when discernible 

 at all, appears as a narrow zone of thin-walled compressed or 

 flattened cells, the long diameter generally between 10 and 15mik. 

 In some cases the transition from wood to cambium and from 

 cambium to phloem is quite abrupt; more usually a gradual 

 transition occurs, at least between phloem and cambium. The 

 different species present no characteristic peculiarities as to 

 the development of cambial tissue. This seems rather to be 

 dependent upon external conditions favorable or unfavorable 

 to rapid growth. 



Xylem. — The primary wood is composed of spiral vessels, 

 fibres and parenchyma, usually in groups projecting into the 

 pith. As a rule, the elements are not greatly lignified, though 

 complete lignification sometimes occurs. The vessels are, in 

 cross section, circular or elliptical in outline, not irregularly 

 polygonal, as are often the pitted vessels of the secondary 

 wood. The spiral thickening is sometimes in a single band; 

 again it may be in two bands running in opposite directions; 

 and in still other cases in two, three or four bands winding in 

 the same direction. The average diameter of the spiral vessels 

 varies from about 12mik. in Kneiffia fruticosa to nearly 25mik. 

 in Onagra biennis. 



The secondary wood is divided by the medullary rays into 

 very many narrow wedges; these wedges, even at their widest 

 portions, are seldom half a dozen cells broad. The wood usu- 

 ally shows considerable regularity in the arrangement of its 



