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 hicolor. 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 

 onea 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, i. e., the new plastids formed are budded off in 



