150 A. H. Graves, 



In lig. 102 appears evidently the beginning of the formation of 

 the dermatogen, at least in the terminal segment at the left. Fig. 

 103 shows the segment lines more irregular, the dermatogen has 

 became more pronounced, and in fig. 104 shows a distinct differ- 

 entiation. A point of interest at this stage is the considerable in- 

 crease in size of all the cells, which are, however, only slightly 

 greater in number than in fig. 103. The embryo here measures 

 about 0.075 mm. in diameter, as against the 0.05 mm. of the pre- 

 ceding one. 



Up to this point the embryo has exhibited a globular form, but 

 in fig. 105, where it measures about 0.085 mm. it has commenced 

 to elongate. It is at this period that the divisions are initiated in 

 the terminal segment, appearing here at the left, which signalize 

 the approach of cotyledonary development. 



It has been shown by Schaffner (1897, II, pp. 263—265) in Sagit- 

 taria that of the proembryo of three cells, the uppermost develops 

 the cotyledon, the middle divides transversely, and of the two result- 

 ing segments the upper develops the stem apex and the lower the 

 hypocotyl, root, and secondary suspensor. The lowest ceU of the 

 three-celled proembryo remains undivided, forming the basal sus- 

 pensor-cell. 



Obviously the primary segments in Ruppia do not all have des- 

 tinies similar to those of Sagittaria. For, to begin with, no second- 

 ary suspensor is here found, and only a minute rudimentary root, 

 as will be shown later. Again, it seems quite probable, although 

 for the reasons stated above no conclusions can be certainly drawn, 

 that the terminal segment produces the stem -apex as well as the 

 cotyledon. Campbell (1897, p. 49j finds such a condition in Zanni- 

 chellia, and the appearances shown in fig. 106 point to such a 

 situation here. Solms-Laubach (1878), as noted by Campbell, has 

 also reported a terminal origin of the stem -apex in some of the 

 Commelinaceae and Dioscoreaceae. This leaves the second segment 

 (which, as has already been indicated, prolDably divides transversely 

 to form the second and third segments, as in Sagittaria) for the 

 development of the hypocotyl, extraordinarily large in Ruppia, and 

 the small rudimentary root adjacent to the suspensor-cell. 



PI. XIII, fig. 105, however, represents the oldest embryo in which 

 the primary segments can be determined with accuracy. In the 

 next figure (fig. 106), it is quite impossible to make out definitely 

 the line separating the terminal segment firom the rest of the 

 embryo, to say nothing of any other segment -lines. Nevertheless, 

 in this embryo, cell divisions are active in the terminal region 



