2 20 
The Ohio Naturalist. 
[Vo!. XI, No. 2, 
dcnced by the increased number of pith-web elements, together 
with the incipient cell rows from which they were derived. The 
unusual number of cortex cells formed rapidly stretches and atten- 
uates the pith-web and inner cortex until the original medulla, 
locally, has been almost replaced by the newer and but slightly 
modified cortical elements. Fig. 3 illustrates the process in a 
more striking manner. This as well as the remaining figures are 
of the same magnification. At this stage periclinal divisions have 
been so rapid that small ridges have been formed in the middle 
of each broad furrow. The enlarging inner cells give the cortical 
structure a fan-like aspect. The inner cortical cells in the middle 
region pass into the middle layer before they reach their maximum 
size due to the rapid development of the cells over them. This 
accounts for the local massed condition of the medulla in the 
splitting region, causing the dark splitting line when the blade is 
viewed by transmitted light. The final result of all this activity 
is the intercalation of a region, made up entirely of new tissue, 
which has no strength to resist wave action and is easily torn apart. 
Fig. 4 shows the lamina at the critical point with the tear partly 
through it. The inner cortex cells of the preceding figure have 
passed into the middle layer condition and the lamina is markedly 
reduced in thickness. When severely whipped the laminae are 
often ripped at their distal ends; the tear if of any depth always 
following the weakened zone of the splitting line (Fig. 1.) 
The wounds formed arc shown in Fig. 5 which in the serial sec- 
tions was taken from the same slide as Fig. 4. By normal activity 
the epidermis and cortex are built out and around the exposed part 
of the medulla (Fig. 6), finally coming in contact (Figs. 7-8). Gen- 
erally the two epidermal layers do not exactly meet and the edges 
of the new laminae show a scar in section. Subsequent to the 
healing, the cells of the inner cortex, overlying the edge of the 
middle layer, do not develop to normal size but become scleren- 
chymatized (Fig. 8). Growth above presses them inward, notice- 
ably bending the hyphae of the middle layer. This condition 
disappears later when the thick walled cortical elements pass over 
to the medulla. 
This method of splitting was observed in several specimens but 
in no instance was any deliquescence or cell disintegration observed 
as reported by MacMillan in his observations on this plant. 
POSTELSIA 
Postelsia and the other Lessoneatae differ from Nereocystis in 
the absence of a long splitting line formed in front of the actual 
cleft. A close inspection shows, however, the presence of a very 
short line indicating that the modification of the inner tissues is 
not begun until the fissure is very near. But for the most part the 
