344 Watson on Structure and Relation of the Plastid. 
and form a complex network within the cell. Fig. XIV 
(sx, and sx,) also (sl, and sl,) seems to show what might 
recall an attenuated condition as seen so completely in 
Psilotum, 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 Zygnema and 
Spirogyra as to general morphology, stainability and rela- 
tionship is suggestive. In the higher plants, from Funaria 
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 additional evidence for and against the present views 
is much to be desired. 
