have also been described, but it is admitted that the forms vary much, and it is very generally conceded that these two geographical race-forms may be united with even less marked varieties into the one species Quercus robur.
The amount of timber produced by a sound old oak is very large, although the annual increment is so remarkably small. This increment goes on increasing slightly during the first hundred years or so, and then falls off; but considerable modifications in both the habit of the tree and in the amount of timber produced annually, result from different conditions. Trees grown in closely-planted preserves, for instance, shoot up to great heights, and develop tall, straight trunks with few or no branches; and considerable skill in the forester's art is practiced in removing the proper number of trees at the proper time, to let in the light and air necessary to cause the maximum production of straight timber.
Oaks growing in the open air are much shorter, more branched and spreading, and form the peculiar dense, twisted timber once so valuable for ship-building purposes. Such exposed trees, other things being equal, develop fruit and fertile seeds thirty or forty years sooner than those growing in closed plantations. The timber itself is remarkable for combining so many valuable properties. It is not that oak timber is the heaviest, the toughest, the most beautiful, etc., of known woods, but it is because it combines a good pro-