96 VEGETABLE SUBSTANCES. 



healthy period of the elm to about one hundred and 

 twenty years. The health of these must have been, 

 however, affected in some degree by the smoke of 

 London. The superb avenue called " The Long 

 Walk" at Windsor, was planted at the beginning of 

 the last century. Most of the trees have evidently 

 passed their prime. The most profitable age of 

 elms, both for quantity and quality of timber, is, 

 probably about fifty or sixty years. The central 

 parts of a tree get indurated, lose their natural 

 sap, and are apt to absorb moisture, by which 

 they soon rot on exposure to the air, long before 

 the dry rot consumes them, shielded as they are by 

 the external parts. The predominance of resin inso- 

 luble in water, and not liable to be acted on by the 

 acids of the atmosphere, is the cause why the pine 

 and the larch are more durable than the silver fir and 

 the spruce. It is possible that the elm is injured by 

 too much humidity in the soil upon which it grows; 

 and that the Dutch elm, which is usually classed as 

 a different species from the common elm, and of 

 which the timber is good for nothing, may be merely 

 the common one debased in the humid soil of Holland. 

 The elm rises to a greater height than the gene- 

 rality of English forest trees, with a foliage at once 

 full and hanging loosely, and thus capable of receiv- 

 ing great masses of light, and of producing " the 

 chequered shade," which imparts such a sparkling 

 beauty to woodland scenes. It is the first tree which 

 salutes the spring with its light and cheerful green; 

 and sometimes very early in the season the branches 

 are dark with innumerable small purple flowers, often 

 as full as the subsequent leafy foliage. At this time 

 (April), the common elms in Hyde-park present this 

 singular and beautiful appearance. The bloom of 

 forest trees is not always annual. 



