80 JOURNAL OF THE [July, 



examination of its anatomy will make more apparent the law of 

 growth and explain somewhat its cause. Beginning, then, with the 

 pith, the structure in no way differs from normal dicotyledons. 

 The cells are hexagonal and arranged in longitudinal rows The 

 faces are approximately equal and finely pitted. The annual 

 growth produces little displacement in the cells — at least no radi- 

 cal distortion. Numerous resin canals traverse the pith. These 

 passages seem due to absorption of the contiguous cell walls, 

 their length varying from a single cell to an indefinite number. 

 The products of assimilation largely disappear by the end of 

 the second year, though in older stems the pith sometimes ap- 

 pears heterogeneous. The pith finally becomes dark colored, 

 partly from the infiltration of dyes from the duramen, but 

 largely from degradation of tissue. At the periphery of the pith 

 the cells grow smaller, thicker, and become elongated longitudi- 

 nally, forming a well-marked medullary sheath. Projecting into 

 this sheath appear numerous bundles of tracheids, the nucleus 

 of the fibro-vascular bundles; a radial section through one of 

 these bundles often showing from ten to fourteen of these small, 

 delicate-walled vessels, with right-hand spirals rising in exception- 

 ally regular and easy ascent. These tracheids appear rarely in 

 the wood. They are at once recognized by their small size 

 (i2 //, with a length of 150 fj), and by the fact that the septa 

 always join the lateral walls at right angles. 



In discussing the elements of the xylem Sanio's classification 

 will be followed. Though somewhat empirical, it furnishes a 

 nomenclature more readily followed and understood than De 

 Bary's or Sachs'. The tissue presents the forms found in ma :-y 

 dicotyledonous woods — pitted vessels, spiral tracheids, libriform 

 tissue, and bundle parenchyma (Figs. 5, 6, 7, and 8). These draw- 

 ings, while ad natu7-am delineated, are intended simply to illustrate 

 the characteristic features of the stem, all repetition of the same 

 tissue and anatomical detail being omitted for clearness. The 

 cambium tissue indicated at M is composed of lenticular cells 

 perhaps more regular than here represented, being distorted 

 somewhat in cutting. The differentiation of the cambium seems 

 to follow no law. Bundles of the various elements are found 

 irregularly associated. Usually arrayed in radial groups in the 

 annual growth, they rarely present the same combination of 



