144 JOURNAL OF THE PLYMOUTH INSTITUTION. 
Plymouth native, growing in our coldest localities ; and Mr. Briggs 
says it is so. 
For usefulness, the Willows are important, their very Latin 
name, Salix, being given from their quick growth. Their growth 
has been proved to render marshes less unhealthy. 
Salix fragilis, the crack willow, which is common with us, will 
grow to a height of 50 or 60 feet, and to a girth at the trunk of 
13 to 15 feet. Its bark contains a large proportion of tanning 
matter, and from it, as well as some other species of willow, is 
obtained salicine, a salt resembling quinine. The wood is con- 
sidered the most useful of the willow tribe, and when cultivated 
in plantations, from their quick growth, the poles yield a large 
profit. Other species common with us are Salix triandra, the 
almond-leaved willow, used for basket-work ; and Salix caj)rea, 
great sallow, useful in the bark as a tonic and for tanning, and as 
a whole for making fences. 
Alnus glutinosa, common alder, is a native, and common in wet 
places ; the wood is soft, very durable, and little liable to warp. It 
is said to be the best wood for piles, for bridges, and for upholding 
river banks ; and also for burning into charcoal to make gunpowder. 
The bark of the young wood is used for tanning leather. 
Quercus robur, common Oak, everybody knows to be very 
common. It is the most important of our forest trees. Of 
the two kinds — the sessile -fruited and the stalked -fruited, or 
pedunculate form — the timber of the sessile-fruited, less common 
with us than the other, is very inferior. A complete account 
of the uses of the British oak would be very lengthy. For 
any work liable to exposure, no wood equals that of the oak. 
Oak-bark is more used around Plymouth, and throughout England 
generally, for tanning than any other commodity. Oak-bark yields 
5 or 6 per cent, of tannin ; all formed in the inner white layers, 
none in the epidermis or exterior. 
Having just mentioned four trees — the birch, the crack willow, 
the alder, and the oak — whose bark contain the tannic acid which 
is found in all astringent vegetables, probably it will be convenient 
here to specify those other Dicotyledons which are noticeable for 
their tanning matter. No Monocotyledon, so far as the lecturer 
knows, yields tannic acid. 
Sarothamnus scoparius, common Broom, we have, common. Its 
young green leaves and twigs are bitter and tonic ; and the bark, 
