Page:An introduction to physiological and systematical botany (1st edition).djvu/61

 Rh of various vessels running for the most part longitudinally; some having a spiral coat, others not. Of these vessels, some in their youngest state convey the sap from the root to the extremities of the branches and leaves; others contain the various peculiar or secreted juices; others perhaps contain air. The whole are joined together by the cellular substance already described.

Linnæus and most writers believe that one of the abovementioned circular layers of wood is formed every year, the hard external part being caused by the cold of winter; consequently, that the exact age of a sound tree when felled may be known by counting these rings. It has even been asserted that the date of peculiarly severe winters may be found in the harder more condensed rings formed at those periods; and moreover, that the north side of a tree may always be known by the narrowness and density of the rings on that side. All this is controverted by Mirbel, chiefly on the authority of Du Hamel, who nevertheless scarcely says enough to invalidate the ancient opinion on the whole. It is very true that there may be occasional