6. =Pópulus monilífera=, Ait. (COTTONWOOD. CAROLINA POPLAR. NECKLACE-POPLAR.) Leaves large, broadly heart-shaped or deltoid, serrate with cartilaginous, incurved, slightly hairy teeth. The rapid-growing young twigs very angular and bearing very large (6 to 9 in. long) leaves. A very large (80 to 100 ft. high) tree, common in the Mississippi valley, but found in western New England and often planted. [Illustration: P. nìgra.] 7. =Pópulus nìgra=, L. (BLACK POPLAR.) Leaves rather large, deltoid, pointed, serrate with glandular teeth, smooth on both sides even when young. Leafstalk somewhat compressed. Buds very sticky. A very variable, large (50 to 80 ft. high), rapidly growing tree with spreading branches. Occasionally planted. From Europe. [Illustration: P. balsamífera.] 8. =Pópulus balsamífera=, L. (BALSAM-POPLAR. TACAMAHAC. BALM OF GILEAD.) Leaves very large, ovate, gradually acuminate, sometimes heart-shaped, finely serrate, smooth, bright green and shining on both sides; leafstalk nearly round; leaves in spring rich yellow. Branches ridged below the leaves; buds large and covered with very fragrant resin. A medium-sized tree, 40 to 70 ft. high, pyramidal in form. Wild in the North and often cultivated. Var. _candicans_, or Balm of Gilead, has larger and more or less heart-shaped leaves (the larger figure in the cut). CLASS II. GYMNOSPÉRMÆ. Plants in which the pistil is represented by an open scale instead of a body with a closed ovary, as in Class I. ORDER =XLI. CONÍFERÆ.= (PINE FAMILY.) As far as the number of species is concerned, this is the largest order of trees and shrubs of temperate and cold-temperate regions. The order is of the greatest importance, both on account of the valuable timber it furnishes and for its resinous secretions, turpentine and resin. GENUS =93. PÌNUS.= (THE PINES.) Leaves needle-shaped, 1 to 15 in. long, almost cylindric, 2, 3, or 5 together in clusters, with a sheath, more or less persistent, at the base. Flowers monoecious, both staminate and pistillate in catkins, usually insignificant and unnoticeable. In spring. Fruit a cone, persistent and formed of more or less woody, overlapping scales. * Leaves usually 5 together in bundles. (=A.=) =A.= Leaves 6 in. or more long, glaucous green and very pendulous 1. =A.= Leaves under 4 in. long. (=B.=) =B.= Cones over 10 in. long, on stalks 3 in. long, pendulous when ripe 2. =B.= Cones 4 to 10 in. long. (=C.=) =C.= Scales of cones thin, unarmed 3, 4. =C.= Scales of cones thick and woody, obtuse, 1 in. broad 5. =B.= Cones under 4 in. long; scales slightly hooked but pointless 6. * Leaves usually in threes, rarely in twos; scales of cones with spines or prickles. (=D.=)