I 



344 Watson on Structure and Relation of the Plastid. 



and form a complex network within the cell. Fig. XIV 

 (sxi and sx„) also (slj and slj) seems to show what might 

 recall an attenuated condition as seen so completely in 

 Psilotnm, but these fibres undoubtedly extend in many direc- 

 tions and constitute a far more complex condition than I 

 have represented. 



Conclusions. 



From a study of the above types it appears that while in 

 the simplest organisms the chlorophyll is diffused through 

 wide areas of the protoplasm, it becomes more and more 

 restricted in the higher type to special bodies, the plastids, 

 that exhibit in most cases a structure closely resembling, if , 



not identical with, the nucleus of the cell in which these \ 



plastids lie. Further, definite refractive threads, that greatly 

 resemble attenuate chromatin, link together the plastids with 

 each other, and with the nuclear membrane. The resem- 

 blance between the pyrenoids and nucleus of Zygncma and 

 Spirogyra as to general morphology, stainability and rela- 

 tionship is suggestive. In the higher plants, from Fitnaria 

 upwards, the connection between plastids and nucleus is evi- 

 dent, while striking resemblances in their finer histological 

 details are undoubted. It seems therefore not unnatural to 

 suppose that plastids primarily represent nuclear differentia- 

 tions of the cell, which have been separated off for the 

 special purpose of metabolizing special fruit constituents, the 

 nucleus in the process being left as the special directive centre 

 of each cell. While it may be extremely difficult to secure 

 positive proof of this proposition, many observational results 

 strongly point in the direction indicated, and the accumula- 

 tion of aclditional evidence for and against the present views 

 is much to be desired. 



