. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. RKSIN PASSAG^:S «3I four contiguous or even coalesccnt reservoirs. The central canal. which is usually small anil not infrequently very narrow, is rather more generally rounded than in previous types. The epithe- lium is very clearly defined and consists of opt- to three rows of thick-walled parenchyma cells, sometimes containing resin, the first row of which are rather small and radially flattened, but in P. macro
. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. RKSIN PASSAG^:S «3I four contiguous or even coalesccnt reservoirs. The central canal. which is usually small anil not infrequently very narrow, is rather more generally rounded than in previous types. The epithe- lium is very clearly defined and consists of opt- to three rows of thick-walled parenchyma cells, sometimes containing resin, the first row of which are rather small and radially flattened, but in P. macrocarpa they arc rather thin-walled. In P. DouKlasii the epithelium is commonly extended on the two sides of the resin canal in such a way as to form a tangentially elongated tract which not infrequently extends beyond and involves neigh- boring medullary rays. In P. macrocaqKi, on the other hand, the epithelium is concentric with the canal, thus forming a tract of aljout equal thickness all around. Such a deviation as is expressed in P. Douglasii constitutes the first evidence of a tendency in development which is fully and frequently expressed in Pinus. Thyloses are of infrequent occurrence, and apjx:ar to be confined to P. macrocari)a where they are few in number and generally rather thin-walled. Parenchyma tracheids are usually not apparent in a transverse section. This results from the frequent IcK-ation of the resin passage in the summer wood, which is not favorable to their development, and from the close resemblance which they bear to the tracheids of the spring wood ; and while such elements form an integral part of the resin cyst, their particular disposition cannot be exactly defined, though there is no good reason for supposing that they differ in this respect from what may be observed in other cases. In a longi- tudinal section the canal is found to be more or continuous, though it presents frecjuent constrictions and is thereby reduced to very narrow dimensio
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