132 A. H. Graves, 
about 0.2 mm. and is about midway between the stages re- 
presented in Pl. IX, figs. 55, 56. This stage is signalized extern- 
ally by the definite appearance of the primordia of pistils and 
stamens. 
Pl. XI, fig. 67 shows the region marked x in fig. 68, being the 
same section at a higher magnification, and reveals the cells in 
one of the thecae of the upper stamen. The initial cells, indicated 
by shading, are distinct from the surrounding tissue by reason of 
their large size, their large nuclei, their dense cytoplasm, and 
especially their strong reaction to stain. In these respects, all are 
essentially alike. 
As is apparent, not only the hypodermal layer, but also several 
of the deeper-lying plerome cells contribute to this group, and 
since they grade off imperceptibly into the sterile tissue below the 
theca, it is well nigh impossible to draw a hard and fast line of 
separation. Thus, it is quite probable that more of the interior cells 
than I have designated are archesporial. 
A point of interest in this connection, and, indeed, an additional 
proof of the identity of these cells, is their previous history. Up 
to about the stage represented in PI. IX, fig. 55, the divisions of the 
meristematic tissue comprising the flower rudiment follow one 
another in rapid succession. From that period on, however, there 
is a slight pause in karyokinesis, with the exception of the divisions 
in -the epidermal region, so that a count of the cells reveals 
practically the same number in PI. XI, fig. 67, as at the end of the 
meristematic condition. But, in the meantime, a_ considerable 
enlargement, cell for cell, has occurred. There is, then, previous 
to the first unmistakable appearance of the archesporium initials 
as shown in fig. 67, a brief cessation of cell division, more or less 
complete, during which occurs a marked increase in their size. 
In one of the Potamogetonaceae at least, namely, Zannichellia, 
more or less uncertainty has always invested the origin of the 
archesporium. Warming (1873, p. 28), long ago, in his study of 
this plant, was of the opinion that the sporogenous cells did not 
arise from a single archesporial layer, but was unable to state just 
how they did originate. Recently, Campbell (1897, p. 41), in his 
study of the same plant, says,—‘ The origin of the sporogenous 
tissue of the anther is not easy to trace, as the archesporial cells 
are at first hardly distinguishable, either in form or contents from 
the adjacent cells. As soon as they are recognizable, there is 
already a group of them whose relation to each other is not 
entirely clear.” 
