36 ON THE BLEACHED WOOD 
or less defined white patches, which more easily catch the eye because 
the other portions of the surface are generally occupied by Crypto- 
gamous vegetation, often of a dark tint. I have taken great pains to 
ascertain the origin of these, but have not always been able to satisfy 
myself about it; they arise, I think, at least from two distinct causes. 
Many are certainly nothing more than patches where the surface has 
been gnawed off by insects, and in all these cases the texture is much 
decomposed, and but few perfect Cryptogams exist upon them, though 
the broken bases of minute perithecia, destitute of everything like fruc- 
tification, are generally visible. But even in these patches different 
portions of the surface are not in the same condition. In one part the 
fibres, when exposed freely to moisture, retain it like a sponge, and have, 
in consequence, an hygrophanous appearance like that of many Agarics 
when wet, while the others retain their opake aspect, whatever quantity 
of moisture is applied. The fibres in the latter case are certainly more — 
decomposed, but not to such an extent as to exhibit any different aspect 
under treatment with chemieal tests. But there is another mode in 
which such spots arise. In planks recently exposed paler patches 
are often visible, simply from the fact that such spots have not been 
attacked by any mycelium. There is probably some difference of che- 
mical condition in these patches, which has not been favourable to the 
immediate growth of Cryptogamic vegetation. A rough plank of Lom- 
bardy Poplar, perfectly free from vegetation when placed in a fence last 
July, has afforded me the means of studying this matter. The whole 
surface is mottled with patches of very pale olive when the surface is 
moist, the olive-tint bemg due to the mycelium of the very Fungus 
-which now exists upon the Arctic Elm. I do not find any Fungus upon 
the whiter portions at present, but the process of bleaching is evidently 
advancing rapidly, though not perhaps so surely as in those which are 
now slightly discoloured from the presence of mycelium. 
In process of time the whole surface of the rail, if the situation is 
- sufficiently damp, is occupied by Algæ and Fungi, and it is then, I be- 
lieve, that the more conspicuous white spots are formed by the abrasion 
of the previously decomposed tissue. Other Fungi are produced occa- 
sionally on such spots, but I do not find in general any second growth 
of Fungi belonging to that peculiar group which first establish them- 
selves on the exposed surface of the wood. A large number of species 
appear to grow in more northern countries, such as Sweden, on bleached 
LI 
