RAMALINA RETICULATA 415 
142 inches in length = 35.7™ 
and 49 of an inch in breadth=7.9™. 
that is, to the same length exactly and to within ,', of an inch 
of the original breadth. 
| Two more points should be noted in connection with the 
| the formation and enlargement of holes; namely, the relative 
| tensile strength of the lichen when dry and when wet, and the 
_ part played by growth. I have made tests of which the follow- 
ing may be taken as the average. A piece of thallus with large 
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_ holes bounded by stout filaments and bands was hung, tip down- 
_ ward, from a hook of small iron wire (not so fine as to cut the 
lichen quickly. This hook passed through a hole in the mesh 
i and the part above appeared to be of average strength. From 
- asimilar hook similarly placed below were suspended weights. 
The breaking strength of the two strands which held the hooks 
Was 150 grams when dry. The lichen was then soaked for fif- 
_ teen minutes in cold water and hung up again, after the surface 
_ Water had been removed. I tried to select for the support of the 
_ Wite hooks two strands of size and position as nearly as possible 
the same as before. The strands broke with a weight of 30 
 §fams, 20 per cent. only of the weight carried when dry. The 
_ xperiment is rude and has evident faults (for instance, the 
- Strands are not of uniform strength under like conditions, hence 
_‘Mtesting dry the strongest strands may be used, thus leaving 
‘solutely, as well as relatively, weaker ones for testing wet; 
and the whole thallus had probably been weakened by weight- 
ing dry, hence it was weaker when wet than it ordinarily would 
“lave been); but it has some value in indicating the quality if 
jf Pot the quantity of difference in tensile strength under the two 
 Onditions, 
F | this experiment indicates the longitudinal tensile strength. 
The transverse tensile strength must be less. Microscopic 
Xamination of sections, and unsectioned but cleared fragments 
i the thallus, shows, as before stated, that the hyphe run 
_ Mainly longitudinally in strands, fewer obliquely, almost none 
pactly transversely, Evidently the transverse tensile strength, 
