412 . BOTANICAL GAZETTE [JUNE 
concludes that the perforations may readily arise by simple 
pulling apart of the generally longitudinally running hyphae, 
adding that the breaks appear in the weakest parts of the 
thallus, namely in the thinnest parts, where the “bark” layer is 
virtually absent and where the spherical gonidia are most 
abundant (they are nowhere uniformly distributed), for in such 
places the hyphe cannot bind them tightly together. This is 
true, but it is not all the truth. Microscopical examination of 
living material shows that the hyphe are not merely in contact 
with the gonidial cells but are attached to them by haustoria 
which penetrate the cells,® and that the hyphe bind the gonidia 
together in fairly coherent masses except when the gonidia have 
divided so rapidly that the young cells are not yet held fast by 
haustoria, though the mass and even individual cells may be 
enmeshed by hyphe. The region of most frequent and most 
rapid multiplication of gonidia is that near, though not neces- 
sarily next to, the tip, and even in fairly old parts, such divi- 
sions of gonidial cells occur, and necessarily occur, if in these 
parts the hyphz are to be adequately supplied with food-furnish- 
ing cells. .To some degree in all parts of the lichen thallus, 
therefore, and most in the younger parts near the tips, there will 
be masses of young and small gonidial cells not yet held firmly 
by investing and penetrating hyphe, and these masses will form 
the less coherent parts of the thallus, which can be more readily 
torn through by the unequal and mainly transverse expansions 
produced by unequal wettings. 
But Lutz, though he mentions the peculiar tips of the 
branches of the thallus of Ramalina reticulata, overlooks the part 
they play in the formation of holes. The tips (figs. 2, 3, 4,5) are 
narrower than the thallus just behind, and are rolled over. A 
section of the tip is crozier shaped (fig. 6). Ifa short branch, 
a few millimeters long, is put convex side down (that is, with 
the apex pointing upward) on a horizontally placed slide, anda 
drop of water put on the upper surface of the branch near the 
apex, the apex will be pushed forward by the longitudinal 
*I shall discuss this in detail in a subsequent paper. 
