416 BOTANICAL GAZETTE [JUNE 
both dry and wet, would be less than the longitudinal. But I 
have shown that the length of a piece of thallus increases 
about 20 per cent. on wetting, whereas the breadth increases at 
least twice as much. _ From these figures we must infer that the 
transverse tensile strength must be much less when the thallus 
is wet than when it is dry. What the proportions are is not 
important. 
These experiments and considerations show that, when the 
lichen thallus is being most strained by expanding and bending, 
it is also becoming weakest, namely when it is wet. 
Growth, whether equal or unequal in different parts, would 
tend to enlarge those holes already formed. The growth of the 
longitudinally-running hyphz, which form the strands bounding 
or between the holes, would lengthen these strands and cause 
them to bound larger holes. Unequal growth in young parts 
where holes had been formed either not all or only in small 
numbers, would produce strains favoring, if not wholly causing, 
the formation of holes at the points of greatest weakness. 
Growth takes place only when and where the lichen is wet and 
therefore mechanically weak. Growth would be unequal if the 
distribution of water in the thallus were unequal. That growth 
is unequal is evident from the fact that only very limited parts 
of the thallus are flat. The water-supply of different parts of 
the lichen will frequently be unequal, growth will therefore nec- 
essarily be unequal, growth strains will be unequal, the thallus 
will be most strained in weakest parts, these weakest parts will 
therefore be made still weaker. The growth of the new gonid- 
ial cells formed by the division of old ones will weaken and 
strain the part of the thallus where they are. If the growth of 
hyphe and gonidia. does not accomplish the formation of 
holes in the weakest parts, it at least facilitates it, and the 
strains produced afterwards by wetting, expanding, and curv- 
ing, will do so. But growth is notoriously slow in lichens, and 
hence it cannot be an important factor in the formation of 
the holes as compared with the purely mechanical effects of 
wetting. 
a a a a a 
LT a Se a ee Re 
