THE WOOD. 65 



cases, on substances which, having performed the 

 work for which they were created, have ceased 

 to be of any use, and that their principal office 

 is to convert dead wood and leaves into rich 

 layers of mould, fit for the growth of future 

 woods and forests. Some of them are so minute 

 that you require a powerful microscope to dis- 

 cover their form ; others attain to a very large 

 size. To the latter class belong the strange-look- 

 ing protuberances on the stump of yonder de- 

 caying Ash- tree. If you examine them closely, 

 you will find that, in their general appearance, 

 they somewhat resemble the valves of oyster- 

 shells : the upper surface is tough and horny, 

 sometimes variegated with beautiful colours ar- 

 ranged in concentric circles ; the under side is 

 perforated with numberless pores, like sponge. 

 This species is often found of large dimensions, 

 sometimes measuring as much as six or seven 

 feet in circumference, and weighing twenty or 

 thirty pounds. Another kind has been known 

 to grow, in a single night, from the size of a mere 

 point to that of a huge Gourd. Its substance is 

 composed of minute cells, which must have multi- 

 plied at the rate of 4,000,000,000 in an hour, 

 or of more than sixty -six millions in a minute. 

 Some species are with greater propriety called de- 

 structive ; these, under the name of Dry Rot, find 

 their way into the timber of houses and ships, 

 which in time they utterly destroy. Some are 

 good for food, as the Mushroom and Champignon. 

 Among these are some which grow underneath 

 the ground; they are called Truffles and Morels, 

 and, in countries where they are abundant, are 

 hunted by pigs and dogs trained to discover them 



