The Structure and Relation of the Plastid. 
(WITH PLATES 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 Dionewa * 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. 1,'p.35. 
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