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3 
1907] CAROTHERS—GINKGO BILOBA 117 
strobili of most other gymnosperms. The succession of structures from 
without to the center of the bud is (2) brown bud-scales, (b) green 
scale-leaves, (c) foliage leaves, (d) ovules, (e) foliage leaves (fig. 2). 
Sections of these ovules show the nucellus enclosed by the integument 
but still entire, no tissue having broken down to form the pollen 
chamber. 
At the beginning of May the ovules average 0.75™™ in length and 
are little if any greater in diameter than the stems that bear them. 
In some cases the nucellar beak protrudes as a tiny papilla from the 
micropyle. Vertical sections (figs. 3, 4) show the presence of a large 
pollen chamber, often containing pollen grains already developing 
tubes. 
SPOROGENOUS TISSUE.—In ovules collected the latter part of April 
and the first of May, the sporogenous tissue has been differentiated, 
and all stages from immature mother cells to complete tetrads are 
to be seen. About at a level with the junction of the nucellus and the 
integument (jigs. 3, 4) is an ovoidal mass of cells (fig. 5) which differ 
from those around them in being larger, having granular cytoplasm 
without vacuoles, and large nuclei with conspicuous nucleoli. All 
these have the appearance of sporogenous cells, although as a rule 
there is only one mother cell (fig. 5). In fifty ovules only one shows 
more than a single mother cell (fig. 6), that single exception possessing 
two. 
The origin of the mother cell from a hypodermal cell could not 
be traced. Nor could it be determined whether the differentiation of 
the mother cell precedes that of the surrounding cells, in which case 
they would be purely tapetal in morphology as well as in function; or 
whether the whole mass is differentiated, the picking out of the func- 
tional mother cell occurring later. If the latter be the case, the mass 
is sporogenous, and the presence of such cells around the youngest 
mother cell observed strengthens the view that such is the case. The 
origin of the two mother cells and the cells between them from the 
same row (jig. 6) is another point in favor of this probability. 
The mother cell is usually slightly below the center of the mass 
(fig. 5), and is distinguishable from the surrounding cells by its 
greater size, more deeply staining wall, and non-granular cytoplasm. 
At first it is non-vacuolate and the large nucleus is near the center 
