RAMALINA RETICULATA 413 
expansion of the thallus, but the youngest part will expand 
More, the apex itself will swell like the rest of the thallus in all 
_ thtee directions (in length, breadth, and thickness) and will 
_ bend upward slightly and then curve backwards somewhat over 
the thallus, thus straining the flat young part of the thallus just 
behind. But if the experiment be modified by placing a similar 
branch convex side down on a drop of water already on the 
de, the apex will curl much more and much more rapidly, and 
s the strain on the young parts just behind will be much 
Because the apex is always narrower than the flat part 
the thallus, and because it is always circinately curved, it 
‘famnot expand so much as the part behind. The young flat 
‘jattis furthermore less coherent than either the closely rolled 
apex or the older flat parts farther back, because of the rapid 
tiplication of the gonidial cells and the consequently feeble 
hment (if any) of the hyphe to them. The apex then, in 
ng over, pulls the young flat part on the convex side, thus 
und the apex, for the narrower circinate, and, therefore, 
“er apex Opposes transverse expansion in the flat thin part 
tlosest to it. Precisely where the longitudinal strain is greatest, 
2 ing transverse ruptures in the looser gonidial areas, there 
transverse strain is also greatest, inducing longitudinal 
in the same gonidial areas. 
Us we see that it is not simply the expansion, or the 
“qual expansion, in three directions which produces the holes 
Me thallus of Ramalina reticulata, although this is the main 
“rin the older parts (where the formation of new holes is 
— 
™ hear the tip, by the folding over of the apex, and the 
fation, owing to the narrowness and circinate curvature 
ex, of the greatest transverse strain in that zone where 
*ngitudinal strain is also greatest. 
