120 
not lengthening in palisade form. This latter was not a fair test, as 
the plant had not been properly cared for. The results obtained 
in the tied leaf were more satisfactory, indicating that palisade 
tissue already formed could not be changed, but that the paren- 
chymatic tissue of the under surface, if exposed to light for a long 
time, would take on the palisade characteristics for the protection 
of the leaf from too intense light. 
The Lunularia, being a plant whose dorsi-ventrality is marked 
by its structural peculiarities, was next investigated. 
It may be well to speak briefly of its normal structure.* Camp- 
bell describes it thus: “ The thallus is made up largely of paren- 
chyma. The dorsal part occupied by a single layer of definite air 
chambers opening at the surface by a single pore, seen from the 
surface they form a network. The thallus is fastened by unicel- 
lular rhizoids from the dorsal surface. One of the methods of 
asexual reproduction is by the so-called gemmae. These gemmae 
are produced in special receptacles upon the dorsal side of the 
thallus, and are crescent-shaped in Lumnularia. These cups are 
specially developed air chambers, which open. The gemmae arise 
from the bottom as papillate hairs; one papilla projects, and a 
wall is formed, separating this projection from the surface of the 
cup ; this outer cell is again divided by horizontal walls until four 
are formed. Each of these four primary cells is divided by a 
vertical wall, the young thallus being but one layer in thickness 
(k, 1, m, n, 0.}, but later walls appear in the central cells parallel 
to the surface, so it is lenticular. As it grows older, two growing 
points are established and lie in a depression so the older gemma 
is fiddle-shaped. The gemma is vertical, therefore has no dis- 
tinction of dorsum and ventrum. The further development de- 
pends upon light. Whichever side touches the ground develops 
rhizoids ; as soon as it becomes fastened to the ground the dorsi- 
ventrality is established.’’"} Sachs says: “Certain cells on both 
convex sides are destined, according to circumstances, to grow out 
into root hairs; if both surfaces are equally illuminated, those 
grow which are able to follow influence of gravity. Zimmerman 
says influence of light is stronger, as if lower surface is illuminated 
* Mosses and Ferns, 45-46, 
+ Physiology of Plants, 526. 
