206 Keegan: The Birch Tree. 
brown cells is developed; in about three or five years later, 
a cubical white thin-walled tissue is interposed in thin layers 
between the zones of the still forming brown cells ; and so it 
proceeds up till the age of fifteen to twenty years, at which time 
the periderm presents the appearance of numerous alternate 
layers (each of about ten rows of cells), of which the two or 
three external ones have thin walls, and the internal ones have 
thick walls; by the decortication of one of these layers situated 
on the outside, the cells of an internal layer are torn or broken, 
and the white resinous matter (betulin) which they contain, 
escapes and plasters the entire surface of the periderm with a 
chalky incrustation. The Birch is a fat tree, z.e. the starch 
completely disappears from bark and wood during the winter, 
and reappears a month or six weeks before the swelling of the 
buds in spring. The wood shows traces of tannin and phloro- 
glucin in the pith and medullary rays, much glucose in summer, 
much oily matter in winter, about 30 per cent pentosan, 0.7 
nitrogenous substance, and o.3 ash, which has 26 per cent 
soluble salts, 35.4 lime, 8.8 magnesia, 10.5 P#O5, etc. The 
trunk bark contains 34 per cent. of a white resin or camphor 
(betulin), 4.8 soluble and insoluble tannin, about 4 gummy 
matter, 60 impure suberin, and 2.3 ash in fresh which has 6.4 
per cent soluble salts, 2.6 silica, 52 lime, with traces of magnesia, 
phosphoric acid, and considerable manganese. The spring 
bleeding period of the Birch, commences at the end of March, 
first from the root, and then step by step towards the crown, 
and lasts some six or seven weeks. The sap contains about 
1.8 per cent. dry matter, of which from 0.8 to 1.4 consists of 
glucose, and the remainder of nitrogenous matters, malic acid, 
and ash mostly of potass. The young spring shoots on dry 
ground are covered with a white resinous secretion in the form 
of vesicular papilla, a circumstance that seems referable to the 
vigorous rapidity of their early growth. 
Leaves.—The mesophyll is composed of one layer of palisade 
cells, narrow and close joined, and a lacunar tissue of equal 
thickness, made up of cells of varying size with air-spaces 
between each; the cuticle is feeble; the cells of the upper 
epidermis are much larger than those of the lower epidermis, 
their lower portion is blocked with mucilage, and there are 
scarcely any hairs on either surface of the adult organ; the 
stomata are small, and have no accessory cells, their number 
per square mm. is about 237; the leaf is only 200 p thick; at 
the base of the petiole are three vascular bundles united into a 
U, higher up, the ends of these branches emit two bundles, and 
Naturalist, 
