Watson on Structure and Relation of the Plastid. 341 



coarsely alveolar nature, within is a very densely staining 

 nucleolus. A number of the plastids are observed to con- 

 tain a single body, each of the same general size, density 

 and staining quality as the nucleolus. Indeed, without other 

 evidence, this is the only fact that would cause suspicion of 

 relationship between the nucleus and the plastids. 



The plastids divide by fission as is shown in Fig. XII 

 (d). where the constriction is just beginning. 



Fiinaria h\groiiictrica. — The thin new leaflets stained in 

 eosin and mounted in balsam were used. The large rather 

 flattened cells near the margin of the young leaves toward 

 the tip were most favorable for study. 



The plastids are large and stain deeply, they are made up 

 of a granular or alveolar structure quite similar to the 

 nucleus. 



The multiplication is by simple fission, and in cells in this 

 position it will be seen by Fig. VII that divisions go on very 

 rapidly. 



It will also be seen that the plastids are joined together 

 by fibres, and that in some cases, for e.xample, Fig. VII (s), 

 a definite system has been formed, i. c, we can easily under- 

 stand how a group of plastids might arise from one parent 

 through successive divisions, and that these might a)l remain 

 joined by filaments which persist as the attenuated connect- 

 ing strand formed when two plastids drew apart. 



Prothallus of Fern (Adiantum). — The plastids are packed 

 quite closely. The nuclei are but little larger than the plas- 

 tids. In structure, both plastid and nucleus are identical, 

 though the nucleus probably stains slightly the deeper. 



The connecting fibres show no definite system, owing to 

 the great number of divisions that take place. They divide 

 by amitosis as in Fig. XVIII (a), (b), fc). 



Fig. XIII shows a peculiar type of plastid occurring in the 

 subepidermal tissue of Pteris bicolor. A limited number of 

 these occur in the different cells. They closely simulate 

 nuclear characters, bands of substance winding in a merid- 



