AMERICAN ^ m , , 
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JOURNAL OF BOTANY^ i!"!!:^ 
Vol. VI July, 1919 No. 7 
THE STRUCTURE OF PROTOPLASM^ 
R. A. Harper 
It may seem to many that there has been little progress in recent years 
in our conceptions as to the constitution of protoplasm. While great 
advances have been made in our knowledge of the molecular constitution 
of various proteins in vitro, we cannot yet feel sure of the adequacy of the 
data so obtained as a basis for conceptions as to the molecular constitution of 
substances found in the living protoplasm. Further, the discovery of such 
bodies as hormones, vitamines, etc., with their far-reaching physiological sig- 
nificance is after all largely a matter of qualitative observations which have 
not yet been brought into relation with our knowledge of the visible proto- 
plasmic cell structures. And yet it seems to me that the new data on the 
chemistry of the colloids and the results of experimental studies in genetics,, 
as well as the more direct observations of cytologists, have been producing 
some very fundamental changes in our conceptions as to the chemical and 
physical characteristics of the cell and protoplasm. 
I have no new theory of cell organization to propose, but desire to bring 
together the data from cytology, colloid chemistry, and genetics which 
bear on this, for all biologists, fundamental problem. That the organiza- 
tion of the cell is the fundamental problem of all biology was never clearer 
than it is to-day. The theory that all organisms are constituted of cells, 
each with its own more or less subordinated life history, and that the func- 
tions of all organs, tissues, etc., are in the last analysis the functions of cells, 
has proved the most illuminating, the most universal in its application,, 
of any viewpoint ever developed in the history of biology. It is so familiar, 
so much a commonplace of all thought and speculation that it seems idle tO' 
dwell upon it, and yet it is certainly a unique situation in science that 
practically the whole developmental series of both plants and animals are 
cellular in organization. A structure so complex as the cell must have had 
an evolutionary history, but the whole evolution of living organisms as 
we know them is the history of the increasing specialization of structure 
and corresponding division of labor between cells whose fundamental 
architecture remains the same. 
^ Address of the retiring president of the Botanical Society of America, delivered at 
Pittsburgh, December 31, 191 7. 
[The Journal for June (6: 217-272) was issued July 21, 19 19.] 
273 
