HYDROCARBONACEOUS SUBSTANCES. 



51 



FIG. 2. 



isms have existed upon the earth, the whole of the free oxygen now 

 present in the atmosphere must have once been liberated from its com- 

 binations by the vegetative process. 



When first produced in the vegetable tissue, starch is in the form 

 of minute, rounded, homogeneous granules. These granules after- 

 ward increase in bulk, reaching 

 a size which varies in differ- 

 ent instances from 2.5 to 50 or 

 60 mmm* in diameter. They 

 often acquire a definite struc- 

 ture, each granule exhibiting 

 under the microscope a series 

 of layers or concentric markings, 

 arranged round a single point, 

 like the scar of a ripe seed, 

 which is termed the " hilum." 

 These characters differ more or 

 less, according to the period of 

 growth of the starch granule 

 and the tissue from which it 

 is derived ;- but they are suflfi- 



ciently well marked in nearly GRAJKS QF POTATO STARCH 



all the varieties which are pre- 

 pared for food or employed in the arts. The starch grains of the potato 

 are among the most characteristic. 



The successive layers of which starch granules are composed differ 

 mainly in their consistency, being alternately harder and softer, thus 

 producing a corresponding difference in refractive power, and an appear- 

 ance of concentric striation. Each granule, furthermore, consists of 

 two substances, intimately mingled in every part of its mass, which 

 rese.mble each other completely in chemical composition, but differ in 

 solubility. These substances are, first, granulose, which may be ex- 

 tracted from the starch grain by boiling water ; and, second, cellulose, 

 which remains undissolved. The granulose is usually much the more 

 abundant of the two, but the cellulose has so marked a consistency 

 that it retains the form and laminated appearance of the starch grain, 

 after extraction of the granulose, though reduced to five or six per 

 cent, of its original weight. 



As starch is the earliest and simplest product of vegetation, it is most 

 abundantly diffused through the vegetable kingdom, and exists, for at 

 least a certain period, in every plant which has yet been examined for 

 it. It occurs especially in seeds, in the cotyledons of the young plant, 

 in roots, tubers, and bulbs, in the pith of stems, and sometimes in the 

 bark. It is very abundant in corn, wheat, rye, oats, and rice, in the 



* The sign mmm. stands for micro-millimetre ; that is, the one-thousandth part of a 

 millimetre. A millimetre is very nearly equivalent to one twenty-fifth of an inch ; 

 and a micro-millimetre, accordingly, is about 2~5%TSo f an inch. 



