MICROSCOPY OF THE COMMON STARCHES. 59 
the inner layers contain an increasing proportion of water. 
At the center there is frequently a small open space of 
much less density, known as the hilum. Starch may be 
seen in situ in the chlorophyll bodies of many leaves in 
process of active vegetation; but it is rapidly changed to 
sugar by the enzymes of the plant, and in that form is 
conveyed to the root, the tuber, the stem, or any other con- 
venient tissue, where it may be reconverted into starch 
and stored for future use. The seeds are particularly 
rich in such reserve material, and with underground roots 
and stems they furnish the principal sources of starch as 
a commercial product. 
2. Refining of Starch.—The processes of starch manu- 
facture are in general simple, involving only the mechan- 
ical separation of the grains from the tissue in which they 
are embedded. Thus, in the production of potato-starch, 
the tubers after washing are ground up in a comminutor 
to break the cell walls as far as possible. The milky 
fluid produced is passed through a sieve with openings 
about !/g9 of an inch in diameter which allow the grains 
to pass while retaining a large proportion of the pulpy 
tissue. The starch is then separated from the impuri- 
ties still present, either by settling, in tanks or in long 
troughs through which it flows very slowly, or by centrifu- 
gal machines; finally it is dried and barrelled for market. 
In potatoes the starch makes up perhaps a fifth of the 
total weight and four-fifths of the dry substance; but in 
the various grains it is found in combination with nitro- 
genous compounds, glutens, etc., which somewhat com- 
plicate the refining process. Corn-starch may be sepa- 
