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AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
Problems Presented by Starch 
These statements hint at a few problems which in turn lead to many 
more. At this place, however, there is no need to do more than to give 
briefly the status of the larger groups of these problems, in order to locate, 
in the whole field of the work on starch, the position of that on the structure 
of the grain. 
About the first problem which attracted the early investigators was that 
concerning the identity of the components, or compounds, which make up 
the starch grain. Starting with Leeuwenhoek (5) in 1719, many investi- 
gators have attempted to solve the problem, but up to the present time, 
the scores of contradictory papers on the subject lead one to conclude that 
the correct answer is not yet firmly established. Many methods have been 
tried and repeated, but there is still considerable uncertainty whether 
starch consists of a single substance or of two or more substances. Perhaps 
it is the ease with which starch becomes modified that is the cause of so 
much uncertainty. 
The place of origin of the starch grain is more certain. It has been 
verified many times and can be considered as settled. Criiger (6, p. 46) 
in 1854 pointed out that all starch grains originate in the protoplasm of 
the cell; Schimper (7) in 1880 reported his discovery of the unstable albu- 
minous bodies which always accompany starch grains ; and Strasburger (8) 
a few years later put on a firm basis the statement of the earlier workers, 
that starch grains are formed inside plastids (protoplasmic bodies) — chloro- 
plasts in the green parts and leucoplasts in those parts which are not green. 
He reports cases, however, in which these bodies are not differentiated 
from the rest of the protoplasm. 
The mechanism of the formation of the grain is not known. Only one 
thing seems certain; that is, that the grains grow only where they are in 
contact with plastids. 
Many of the chemical and physical properties have not yet become 
fully established. About the only one which seems quite satisfactory to 
the chemists is the percentage composition. They have accepted CqHiqOs 
as reported by Guerin Varry (9) in 1834, but whether a dozen or a hundred 
of those groups constitute a molecule is not at all certain. The formula, 
therefore, is written (CeHioOs)?^, and the molecular weight is often written 
''n 1 62. 1." The solubility of starch is uncertain and the specific gravity 
is variable. The color reaction with iodine solution varies greatly with the 
conditions of the experiment. The reactions with acids and enzyms lead 
into a large field of uncertainties, which, however, have little or no direct 
bearing on the present work. 
Structure of the Grain 
The physical structure of the grain has been a subject of much discussion 
during the past hundred years; and since this paper deals with one of the 
