342 Watson on Structure and Relation of the Plastid. 
ional or parallel manner about the plastid and staining very 
like chromatin. Possibly this indicates a trace of nuclear 
character occurring in certain specialized plastids of the 
ferns. 
Fig. VI represents plastids and nucleus from a deeper 
level, also in Pteris bicolor. The plastids here stain densely, 
and show decided tendency to grouping by connecting 
threads. The nuclei are of finely granular type and bear 
from one to several nucleoli. 
Psilotum.—This plant proved to be one of the most advan- 
tageous of the materials used. Here, no doubt, a large part 
of the success was due to varied methods of killing, mount- 
ing, etc. I found, as stated under “Methods,” that Flem- 
ming’s fluid was the most satisfactory killing agent, the 
specimens being subsequently stained and mounted in both 
acetic acid and in balsam. Of these the acetic acid proved 
clearest as to detail. 
The plastids are of two sizes, the large deeply staining 
ones occurring in the subepidermal tissue, and the smaller 
less dense type, that is scattered through the deeper cells, 
making up the greater portion of the substance of the plant. 
Both nuclei and plastids present the same rather coarsely 
alveolar structure. 
The subepidermal cells for about four, five and six layers 
are quite crowded with plastids. In these cells the struc- 
tures are so obscured that I have made my drawings from 
the deeper, more elongate cells with fewer plastids. The 
nuclei in both types of cell are of large size, of an alveolar 
nature and show a distinct bounding membrane. 
Figs. IX and XIV show the plastids grouped in the deeper 
tissues. Here is present the arrangement that is so evident 
in the mosses, indeed it seems more striking in this type 
than in any other examined. 
Fig. VIII is an enlarged drawing of the nucleus of the 
cell shown in Fig. XIV (a). The group of plastids is a 
diffuse type, 1. e., the new plastids formed are budded off in 
