2 24 
The Ohio Naturalist. 
[Vol. XI, No. 2, 
a few hypodermal and outer cortical cells begin to divide rapidly 
forming a wedge-like mass, which tears the epidermis apart and the 
process which succeeds in dividing the lamina is begun (Fig. 16.) 
The cells exposed by the parting of the epidermis become passive 
and subject to the tearing tendencies of the rapidly expanding 
tissue beneath them. They are separated and in this manner the 
cleft is carried clear through the lamina. So great is the meris- 
tematic activity that before the cleft reaches the pith-web this 
layer locally has been entirely replaced by dividing cortical ele- 
ments, through which the cleft is propagated. The final separa- 
tion of the last thin walled cortical cells is of course mechanical. 
By the continual extension of this cortical activity distally, the 
whole lamina is finally divided, while proximally, the separation 
is carried some distance down the stipe by the same sort of activity 
except the meristematic wound tissue is formed in larger masses 
and the cleft advances in a more irregular manner. 
In healing, the superficial cells of the exposed wound tissue 
are transformed into epidermal elements. There is however a 
tendency to close the wound as previously described, by the 
crowding or pressing around of the tissue adjacent to it. 
Material containing clefts of proper age to show the transition 
stages, by which the initial gelatinization process gives way to the 
secondary process of cortical activity, was not available so this 
nteresting phase of the problem cannot be taken up in the present 
diiscussion. 
DICTYONEURON. 
In Dictyoneuron only the method of advance of the older cleft 
was studied, as the collection contained no material showing the 
incipient or perforation stages. The process involved in the 
advance of the cleft was essentially the same as that in Macro- 
cystis but the cortical meristem is more definitely localized than 
in that genus and only occurs at first on one side of the medulla. Fig. 
17 shows a section of a young lamina in which a split 5 mm. in 
length was present. The half of the section not shown was nor- 
mal like the region at the edges of the drawing. Cell division 
and growth in the cortex has resulted in the formation of a mass 
of tissue which presses slightly into the pith-web. When this mass 
has become somewhat more extensive than that figured, a few cells 
near its center begin dividing very rapidly and build up a new 
secondary mass within the first (Fig. 18), which pushes out the 
older cells on all sides of it, notably below into the pith-web. 
On account of this rapid internal division, the original epidermis 
is pulled apart from a to b and the beginning of the cleft has been 
started by the wedging action of the ball like mass of new tissue. 
This cleft shown at Fig. 18, c, next enters the central mass and 
passes rapidly to its center. After the development of the cleft. 
