The third section of the family Abietinæ is formed by the genus Pinus, of which the Scots fir, or Scotch pine (Pinus sylvestris), is the type. The Abietinæ of this genus are distinguished by their foliage. There are two kinds of leaves--the primitive ones, which are little, scale-like, green up-growths closely scattered on the young branches; and the secondary ones, which are long needles carried as a tuft or fascicle on a very stumpy branchlet. These tufts of needles are persistent (that is to say, are not shed yearly), and differ from those of the larches and cedars in consisting of but few needles in a tuft, the number being characteristic of different species, some having five, others three, others two, and the American Pinus monophylla having only one. The general shape of these trees is not tapering like the spruce with unforked trunk, but they usually shed the lower branches as growth goes on, and present in most cases a trunk carrying an umbrella-like expanse of foliage-bearing branches, or several such expanses. The scales which form the cones in the genus Pinus are (with few exceptions, such as the Weymouth pine) not flat and flexible, but are thickened, swollen, and even knob-like and wooden at the exposed part, which is armed with a weak or a strong prickle (see Figs. 39, 40, and 41). The cones do not ripen until the end of the second or third season; they may be, according to species, erect, pendulous, or horizontal, and vary in size in different species. In some they remain closed on the trees for an indefinite period (even fifteen or twenty years), until opened by the heat of a forest fire or of an exceptionally hot season.