144 MINUTE STRUCTURE OF THE STEM. 



somewhat oblique thereto. The degree of obliquity is generally 

 from 4 to 5, but it is sometimes much higher than this ; for 

 instance, 10 to 20 in horse-chestnut, 30 in Syriuga vulgaris 

 (Lilac), 40 in Sorbus aucuparia, and 45 in Puuica Granatum. 

 411. Density of wood. Owing to its greater firmness and 

 smaller amount of putrescible substances, heart-wood is economi- 

 cally of far greater value than sap-wood ; and hence nearly all 

 determinations of density, strength, etc., are made upon it, 



/3. The radial rows of the smaller ducts are relatively narrow and 

 for the most part isolated tangentially : Quercus bicolor, ses- 

 silitlunt, Iberica, grosseserrata, castaneifolia, pedunculata, 

 Thomasii, undulata (var. grisea), Mongolica, macranthera, 

 heterophylla. 



y. The radial rows of the smaller ducts are very narrow, and the 

 ducts differ somewhat in width. The large ducts are in groups 

 in the concentric circles : Quercus lobata. 

 b. With thick-walled ducts. 



a. The large ducts in the concentric circles are indistinctly grouped, 

 while the small ducts are crowded in narrow radial rows : 

 Quercus rubra and the var. ? Texana. Quercus tinctoria. 

 /3. Large ducts, as in the previous group. The radial lines of the 

 smaller ducts wide, and the ducts themselves visible to the 

 naked eye : Quercus imbricaria, hypoleuca, laurifolia, Kelloggii, 

 palustris, talcata, Catesbaei, aquatica, nigra. 



7. With distinct radial grouping in the circles of the larger ducts of 

 the spring wood. The radial rows of smaller ducts narrow and 

 straight. The small ducts visible to the naked eye : Quercus 

 Cerris, serrata, Phellos, coccinea. 



B. Having thick-walled ducts of one kind, and these arranged in radial 

 rows or groups. The annual rings are not distinct to the naked eye, 

 and are denned chiefly by the thick-walled wood-cells of the outer 

 layers of the autumn wood. They are easily made out under the 

 microscope. 



a. The radial rows of ducts are for the most part wide : Quercus 



virens, oblongifolia, chrysolepis, rngosa, Ilex, coccifera, Calli- 



prinos, lanuginosa, paucilammellosa, glabra, Burgeri, gilva, 



thalassica. 



/3. Radial rows of ducts mostly narrow : Quercus Suber, agrifolia, 



glauca. 



II. The wide medullary rays appear under the microscope to be somewhat 

 interrupted by wood-cells, so as to appear like groups of narrower 

 rays : Quercus dilatata. 



The principal kinds of wood-cells in oaks, according to the nomenclature of 

 Abromeit, are : first, the " pointed," of which there are two varieties, the septate 

 and the unseptate ; and, second, the "blunt," which are of comparatively wide 

 caliber, and have thin walls. The length of the pointed cells in an average of 

 171 measurements was found to be 1.224 mm. ; that of the blunt cells only 

 .1 mm. Besides these two chief kinds, there are transitional forms of every 

 sort. 



