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H. W. RICKETT 
in the size and number of the lobes, than is the case under natural condi- 
tions. The thallus, instead of being a simple small plate, the edges of which 
are cut into more or less crowded and overlapping lobes, as is the case in 
nature, took on the form of a central thick, distinct axis bearing leaf-like 
lobes irregularly on either side, and attaining a considerable size. The 
involucres also grew longer and were more cylindrical in form, the mouths 
often flaring out and becoming undulate. These peculiarities are even 
more noticeable if the plants are grown in petri dishes on filter paper mois- 
tened with a nutrient solution (fig. 70, PI. XII). Campbell (2), working, 
according to Miss Haynes (8), on a mixture of S. texanus and S. cristatus, 
described similar forms occurring in nature under exceptional conditions 
of moisture and shade, but in these cases the production of sex organs was 
partly arrested, whereas in the plants I studied rather the reverse was true. 
The Douins (4, 6), studying S. Michelii and S. texanus, also mention the 
growth of abnormal plants under cultural conditions. According to the 
description of C. Douin (4), the plants usually possess in nature three large 
lobes with a mall "middle lobe" in each of the two apical notches. The 
figures of Allen (i), however, made from living plants grown under natural 
conditions, show a less schematic and more luxuriant growth. 
Leitgeb (9) says that the lateral segments of the apical cell continue 
for a time to cut off dorsal and ventral segments in the same manner as the 
apical cell itself, but sooner or later a lobe cell is formed, which grows out 
by vertical divisions into one of the lobes. This is the same condition as is 
sjiown by longitudinal vertical sections through the apical region (figs. 
38-54). In such a case it is extremely difficult to determine whether 
there is a group of apical cells present, or a single apical cell, which has 
formed new apical cells to either side in preparation for a forking of the 
growing region. My figures show several small cells, having the shape 
usually described for the four-sided type of apical cell, grouped together in 
the apical region {a, figs. 38-54) ; and, in the case of one of the apical 
regions illustrated, several such groups are present, each two consecu- 
tive groups separated by a young lobe (/, figs. 38-48). The presence of 
several groups of initial cells indicates presumably that the original group 
had divided several times in close succession, the intervals between the 
groups being occupied by lobe cells. This is usually the case under cultural 
conditions, and dichotomy is correspondingly rapid. An examination of 
the plants under a hand lens confirms this interpretation of the figures. 
The apical region is often very wide, and there are usually from one to four 
small lobes in it. The fact that each group of initial cells consists of cells 
all alike and all having the appearance of four-sided apical cells, seems rather 
to favor the idea that growth is not due to a single apical cell but to a group 
of initials. It may be, of course, that the condition depends upon the 
environment, and that in nature a single distinct apical cell exists, while in 
culture this single cell is multiplied without a corresponding forking of the 
growing region necessarily resulting. 
