132 .A. H. Graves, 



about 0.2 mm. and is about midway between the stages re- 

 presented in PI. IX, figs. 55, 56. This stage is signalized extern- 

 ally by the definite appearance of the primordia of pistils and 

 stamens. 



PI. 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 p5int 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 PL 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 PL 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." 



