6 TRANS. ST. LOUIS ACAD. SCIENCE. [166 
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. Canarzensés has regularly paren- 
chymatous ducts, but sometimes they are connected with the 
epidermis by a very thick bundle of strengthening cells (of which 
presently) ; 7. Zariczo has normally parenchymatous ducts, but 
in a specimen of var. Pyrenazca 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 
Teda, and also in P. pungens and filifolia, 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 Strodus section; in P. Pseudostrobus and P. Jilifolia they 
are very imperfectly developed. The strengthening cells are prin- 
cipally found under and close to the epidermis (whence the name 
hypoderm cells) either ina 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. resz- 
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- 
dies. Their presence and position is not absolutely constant, but 
