202 WOODY PLANTS OF MASSACHUSETTS. 
Trees of the birch family are nowhere of more importance, 
variety and beauty, than in this country. There are but two 
genera:* 1. The Birch, known by its thin and delicate leaves, 
and bark made up of strong, horizontal fibres; and, 2. The 
Alder, known by its thick leaves, polished, dark-colored bark, 
and woody, cone-like, persistent strobiles. 
V. 1. THE BIRCH. BETULA. Tournefort. 
This genus has its sterile aments, made up of imbricate scales, 
arranged in threes, with twelve stamens placed beneath the 
middle scale, and its fertile aments of three-lobed scales. The 
ovary is much compressed, crowned with two styles, and divid- 
ed into two cells, the ovule in only one of which comes to ma- 
turity. The seed-vessel is a samara with thin, winged, mem- 
branous borders, like that of the elm. ‘The buds are sessile, 
covered with imbricate scales, and contain the Icaves folded 
together, and overlying each other. The leaves are alternate 
on the growing branches, and in pairs elsewhere; on the canoe, 
the gray, and some other birches, they are sprinkled with glu- 
tinous dots when young. The sterile aments make their appear- 
ance in July, remain unprotected on the branches through the 
autumn and winter, and expand, before the leaves, with the ear- 
liest warmth of spring. The scales of the fertile aments detach 
themselves easily, and fall from their stems, which are always 
undivided. This genus contains not far from twenty species, 
of which nine or ten are found within the limits of the United 
States or its territories. ‘The rest belong to the north of Europe, 
except one found in Japan, one in Terra del Fuego, and some 
which grow among the mountains of Central Asia. 
No trees are more distinguished for their hight and feathery 
* The Clethropsis, a plant of the interior of Asia, discovered by V Jacque- 
mont, and described from his specimens, by Cambessedes, must take its place, ap- 
parently, between the birch and the alder, and nearer to the latter. See Voyage 
dans U' Inde, par Victor Jacquemont, Tome IV, p. 158, Plate 159. 
Whilst this sheet is passing through the press, I learn, from the eleventh volume 
of Spach, Histoire des Végétaux, which I had not before seen, that he has placed 
Clethropsis 1n this family. He has also made two other genera, from species of 
Betula and Alnus. 
