414 BOTANICAL GAZETTE [JUNE 
Such experiments as those just described can be performed 
on intact thalli or larger fragments, either in the laboratory 
or in nature, quite as well as upon small pieces on the stage of 
the microscope. The dry thallus may not be able to retain the 
whole of a drop of water, but some of it will be held by the 
gelatinous outer part; there expansion will take place, and, if 
enough water has been held, curvature also. The nearer the tip 
the point is upon which the drop falls, the thinner it will be, the 
more prompt and the greater will be the expansion, the more 
pronounced the curvature, the greater the strain, the less cohe- 
rent the structure of the thallus, the greater the likelihood of 
ruptures occurring in the weakest areas. Except when rain falls 
unaccompanied by any wind, and except in fog, the thallus is 
not likely to be uniformly wetted and in the former case uni- 
form wetting would not be accomplished immediately. So in 
nature the conditions of expansion and curvature, as demon- 
strated by experiment, are normally realized. 
Lutz says that drying after wetting brings about a change in 
the shape of the thallus, the length being greater, the breadth 
less, but the area the same as before the water was supplied ; 
and that these contractions and changes in shape produce new 
and enlarge already existing holes. If such changes normally 
took place on drying, holes would result; but they do not take 
place. Lutz’s method of experimentation is probably to blame ek 
his conclusion. He used such small pieces of lichen that his 
experiments were conducted on the slide, under a magnification 
of thirty diameters. The moistened gelatinous matter on the 
surface of the lichen would naturally adhere to the glass some- 
what, enough at least to prevent perfect contraction to the 
original form, and, as his fragments were several times longer 
than broad, there would be most adherence and least contraction 
longitudinally, which was what he found. I allowed the frag- 
ment which I had used for determining the expansion on wet- 
ting, in the experiment above reported, to dry quite wee of 
such adherences by hanging it up. It contracted within five 
hours to 
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