1 66 TRANS. ST. LOUIS ACAD. SCIENCE. 



are mostly two peripheral dorsal ducts, a third upper parenchy- 

 matous one ; in P. Bungeana^ which generally has numerous 

 peripheral ducts all around, occasionally a single lateral paren- 

 chymatous duct is observed ; P. Canariensis has regularly paren- 

 chymatous ducts, but sometimes they are connected with the 

 epidermis by a very thick bundle of strengthening cells (of which 

 presently) ; P. Laricio has normally parenchymatous ducts, but 

 in a specimen of var. Pyrenaica from the Pyrenees in Herb. Cos- 

 son I find them sometimes almost touching the epidermis cells, 

 and therefore easily mistaken for peripheral. In P. rigida and. 

 Tceda^ and also in P. pungens and Jilifolia^ which all have 

 normally parenchymatous ducts, I occasionally have observed a 

 number of smaller accessory ducts close to the sheath of the ves- 

 sels. In pines with very slender leaves it is sometimes difficult to 

 discover the ducts, and in some forms they are, I believe, really 

 absent, especially in cultivated specimens. Such may give us 

 some trouble in their classification. 



A peculiar element in the structure of the pine leaves are cer- 

 tain cells which had been formerly named '' hypoderm cells"; 

 but as they also occur in other parts of the leaf apart from the 

 epidermis, they more appropriately receive the name of strength- 

 ening CELLS. They are thick-walled, elongated, colorless cells, 

 much larger than the bast and wood cells, generally of the diame- 

 ter of the epidermis cells, rarely a little larger, often smaller, and 

 always smaller than the cells of the parenchyma. They give to 

 the leaf its rigidity, and are most abundant in the most rigid pine 

 leaves ; in the softer more flaccid ones they are almost entirely 

 wanting. Thus they are scarce or entirely absent in some species- 

 of the Strobus section ; in P. Psendostrobiis and P. Jilifolia tliey 

 are very imperfectly developed. The strengthening cells are prin- 

 cipally found under and close to the epidermis (whence the name 

 hypoderm cells) either in a continuous layer or mostly in bun- 

 dles, interrupted by the lines of stomata ; they are generally 

 most abundant within the angles of the leaves. Sometimes they 

 surround the ducts, and in all the species allied to P. rest- 

 nosa and P. sylvestris they are found only there, and not or 

 scarcely at any other places. In some species these cells also 

 occur within the sheath, above and below the fibro-vascular bun- 

 dles. Their presence and position is not absolutely constant, but 



