396 



Verfetahle Plnjsioloriy. 



It Is when thus collected in quantity that starch is of most con- 

 sequence in the eyes of the agriculturist, but the physiologist is 

 ilo less interested in its presence in smaller quantities in the cells 

 of actively vegetating tissues, since it here evidently plays an 

 important part in the general history of the nutrition of plants. 



The characters of starch are best examined in some of those 

 structures just referred to as containing it stored up in large 

 quantities in their cells. If we place beneath the microscope 

 thin sections of a potato-tuber, we perceive that its bulk is corn- 

 Fig. 12. 



Starch granules of the potato : a, various forms in the nntural condition, the larger ones charac- 

 teristic; 6, a granule heated so as to cause a pecling-off of some of the concentric coats; c, gra- 

 nules swollen by heating in water; d, granules softened and " blovvn-out " by more violent 

 action of liot water; c, granules in course of solution, from a growing tuber.' Magnified 400 

 diameters. 



posed of membranous sacs or cells, containing a quantity of 

 rjranules of very varied sizes, but of a form tolerably definite in 

 all the larger examples, (See fig. 4, p. 75 of vol. xvii.) The 

 smallest of these starch-yranules are globular, the largest are 

 more or less regularly egg-shaped, and intermediate forms are 

 found in the intervening sizes (fig. 12a). In all but the smallest, 

 concentric streaks may be observed, indicating a lamellated struc- 

 ture, the lamella* being proportionately more numerous as tlie 

 granules are larger (and older) ; this structure may be roughly 



