The Structure and Relation of the Plastid. 



{WITH PI, ATMS XXVI, XXVII.) 



By Cassius H. Watson, B. S. 



In the study of the morphology and physiology of the 

 plant cell, great advances have been made during the past 

 few years in the investigation of the structure, division and 

 distribution of plastids in different plants. The old defini- 

 tion, "plastids are differentiated portions of the protoplasm, 

 which, like the nucleus, are not formed de novo, but multiply 

 by division," is so far satisfactory, and may serve as a start- 

 ing point from which to investigate other questions bearing 

 on the plastid. The above definition gives no suggestion as 

 to the genesis of the plastid, and it is to the study of this 

 question that the present paper is devoted. The study was 

 undertaken at the suggestion of Professor Macfarlane, who 

 alike in his paper on Dionoea * and in his graduate lectures 

 has shown that some plastids are connected with the cell 

 nucleus by chromatin threads, and exhibit a minute structure 

 that seems to be identical with the nucleus of the cell to 

 which they are organically attached. 



From the investigations of Schimper, Meyer and others 

 we now know that plastids exist in the cells of the embryo 

 plant even before the seed-coat has ruptured. All evidence 

 is in favor of the view that these are descended from the 

 cells of the parent tissue, out of which the sex-cells were 

 organized ; a similar relation seems to hold true for the sex- 

 cells and spore-cells of the lower plants. 



But in order to ascertain regarding the genesis of the 

 plastid, it seemed possible that a comparative study of them 



♦Botanical Contributions, University of Pennsylvania, Vol. i,'p.3S- 



(336) 



