Page:Popular Science Monthly Volume 15.djvu/544

528 by rot, and that, within one year after the house was erected, all the basement floor was in a state of premature decay. In cases of dry-rot, where the mycelium passes through substances from the external surface, it separates into innumerable small branches; when it proceeds from slime in the fissures of the earth, the mycelial fibers shoot in every direction and are very much tangled. Arising from the roots of trees, they look at first like hoar frost, but soon show regular toadstools. When they grow in very damp situations, they feel fleshy and extend equally around a circular space which they wholly cover unless obstacles interpose.

Excessive damp is unfavorable to this fungus, and its growth is more rapid in proportion as the situation is less damp, until the proper point for the growth of vegetation is reached. When the fungus extends to dry situations, its effects are more destructive to the timber on which it grows: it is very fibrous, and in part covered with a light brown membrane perfectly soft and smooth. It is often of great magnitude, projecting from the timber in a white spongeous excrescence, on the surface of which a profuse humidity is frequently observed. Sometimes it forms only a fibrous, thin-coated, irregular web on the surface of the wood. Excrescences of a fungiform appearance are often protruded amid those already described, and are evidences of a very corrupt state at the spots whence they spring. Sometimes they arise in several fungiforms, each above the other, without any distinction of stem; and in some corrupt states the small acrid mushroom is generated.

But there are two or three species of fungi that are chiefly concerned with the process of dry-rot. The Merulius lachrymans (often called the dry-rot) is a most formidable enemy of timber. When the section of a piece of wood attacked internally by dry-rot is examined through a microscope, and minute white threads are seen interlaced and matted together all through its substance, and when this cottony texture effuses itself over the surface of the timber, and in the center of it a gelatinous substance forms which gradually becomes tawny and wrinkled and sheds a red powder on the white, downy surface, you have the Merulius lachrymans. But, long before this last stage of growth is reached, the interior of the wood has perished. As soon as the cottony filaments are seen upon timber internally affected, we may be sure that an apparently solid beam may be crumbled to dust between the fingers. In his botanical description of this plant, Dr. Greville says it is "soft, tender, at first very light, cottony, and white; veins appear, at length, which are of a fine orange or reddish brown, forming irregular folds, usually so arranged as to appear like pores, but never anything like tubes, and, when perfect, distilling drops of water." Hence the term lachrymans. The folds or pores here spoken of are the reproductive portion of the plant. They are covered by the hymenium or spore-bearing membrane, which sheds its red powder