REVISION OF THE GENUS PINUS. 363 
sometimes occupy an abnormal position. Thus I find occasionally in some Strobi, especially 
in P. excelsa, where there are mostly two peripheral dorsal ducts, a third upper parenchyma- cant, 
tous one; in P. Bungeana, which generally has numerous peripheral ducts all around, occasion- 
ally a ais lateral parenchymatous duct is observe . Canariensis has regularly parenchymatous 
ducts, but sometimes they are connected with the percieiae 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. Cosson I find them sometimes almost touching the epidermis 
cells, and therefore easily mistaken for peripheral. In P. rigida and Teda, and alsoin 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 vessels. 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 is certain 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 STRENGTHENING CELLS. They are thick-walled, elon- 
gated, colorless cells, much larger than the bast and wood cells, generally of the diameter of the 
epidermis cells, rarely.a little larger, often smaller, and always smaller than the cells of the paren- 
chyma. 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. Pseudostrobus and P. filifolia they are very 
imperfectly developed. The strengthening cells are principally found under and close to the epider- 
mis (whence the name hypoderm cells) either in a continuous layer or mostly in bundles, interrupted 
by the lines of stomata; they are generally most abundant within the angles of the leaves. Some- 
times they surround the ducts, and in all the species allied to P. resinosa 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 bundles. Their presence and position : 
are not absolutely constant, but may be relied on to some extent for diagnostic purposes. Thus [167] 
the quinate smooth-edged leaves of P. flewilis and P. Balfouriana, which would be difficult to 
distinguish without their cones, may be readily recognized by the strengthening cells, which in the 
latter surround the more closely approximating ducts, while in the former the ducts, widely apart 
from one another, are destitute of these cells. 
The PERSISTENCE of the leaves is very different in different species ; in P. Strobus and others 
they fall in the autumn of the second year ; more commonly they last to the end of the third year; 
in some species, e. g. P. Banksiana, they do not fall before they are 4, 5, or even 6 years old; in 
P. Balfouriana, or at least in var. aristata, I have seen them persist 12 to 14 years. When the 
leaves persist only a short time and are long, and the annual growth of the axis is short, they form 
brushes or tassels (P. australis) at the end of the branchlets, but where they are short and persist 
long (P. Balfowriana) they give the branchlets that “ fox-tail or bottle-brush ” appearance of which 
travellers speak. In young and vigorous trees the leaves are apt to persist longer than in old 
ones. 
In exceptional cases and as a monstrosity the leaf-bundles become proliferous, the branchlet 
which bears the secondary leaves elongating and forming a regular branch. 
The pines are moncecious trees which bear their male and female flowers generally on different 
branchlets, the male commonly on the lower, the female frequently on the upper part of the tree ; 
sometimes both are found on the same axis, the male below, the female above. 
The MALE FLOWERS are borne on the lowest part of the year’s shoot, in the axil of bracts, either 
crowded together in a kind of a head or elongated in a spike; the axis usually continues to elongate 
during or after flowering and makes a leafy branch, which in its continuation in succeeding years 
