124 WOODY PLANTS OF MASSACHUSETTS. 
been added throughout the whole forty feet, though not much, 
probably, will have been added to the height. Now, as the 
srowth must be estimated by the squares of the diameters, the 
solid wood in the lower part of the trunk will have increased 
in the proportion of 100 to 64. In the next ten years, it will 
increase in the proportion of 144 to 100; 1n the next ten, in that 
of 196 to 144; and in the next, in that of 256 to 196. ‘The 
numbers after them will be 324, 400, 484. The successive ad- 
ditions, in periods of ten years, will be as the numbers 36, 44, 
52, 60, 68, 76, 84, 92, 100. A tree of thirty years, therefore, in 
ten years, will increase 56 per cent.; in the next similar period, 
68 per cent.; in the third, 79; in the fourth, 93; in the fifth, 
106. That is, an oak of eighty years of age grows more in ten 
years than it did in the first thirty ; and an oak of one hundred 
and thirty years, more than in the first forty. When, therefore, 
it is desirable to keep the growth for tumber, the process of 
thinning may be continued with strict economy, as the increase 
of the thirty or forty trees left on the acre, will counterbalance, 
in a great degree, the loss in numbers. Some acres, in every 
large forest, should be thus left, for the use of the ship-builder. 
Those species of oak most analogous to our white oak, are 
known, in Europe, to continue to grow and flourish for cen- 
turies. There are oaks in Britain, which are believed to have 
been old trees at the time of William the Conqueror. Some 
are known, which are supposed to be one thousand years old. 
The number of species of the oak known to botanists, 1s very 
great. In 1823, the whole number was one hundred and thirty ; 
(Dictionnaire Classique d’ Histoire Naturelle;) since which time 
a considerable number has been added. Loudon estimates them 
at present at one hundred and fifty. This number is probably 
over-stated, as many that are considered species, will doubtless 
be found to be varieties. Sprengel enumerates more than one 
hundred oaks, the larger part natives of this continent. The 
elder Michaux described twenty, the younger, twenty-six, as 
natives of North America. Pursh described thirty-four as be- 
longing to North America. Nuttall, in 1817, mentioned thirty- 
two as belonging to North America. Eaton describes thirty-six 
as found north of the Gulf of Mexico. Beck, twenty-three, as 
