220 



HARDWICKE'S SCIENCE- GOSSIP. 



and most delicate part of the granule, and contain- 

 ing matter of the most recent formation. In the 

 process of drying the nucleus cracks, and being 

 filled with air, frequently appears as dark lines 

 under the microscope. The concentric or eccentric 

 rings, as the case may be, were formerly supposed 

 to be markings on the external membrane, but it is 

 now known that they denote a system of lamina- 

 tion of cellulose. 



Fig. 119. Section of Potato, showing starch-grains in situ. 



The mode by which starch granules increase in 

 size has long been a problem, and even now con- 

 tinues to be a disputed point among microscopists ; 

 one party, including Henfrey and Griffith even in 

 the last edition of the " Micrographic Dictionary," 

 asserting that the granules increase by the superpo- 

 sition of external layers from within, so that the 

 outermost layers would be the youngest, while 

 another party, including such eminent authorities as 

 Sach and Nagelli, assert that the layers are formed 

 internally, the older ones expanding to make room 

 for the younger. The latter theory, or growth by 

 intussusception, seems most in accord with the 

 most critical investigations on the subject ; for if 

 the external layers were added by deposition, the 

 outer layer would be the least dense, whereas it is 

 the densest part of the granule, and the nucleus 

 the softest. Again, when two nuclei occur in one 

 granule, a not uncommon circumstance, the strongest 

 growth always takes place in a line where the nuclei 

 meet ; whereas, growth by addition to the exterior 

 would probably tend to make the union of the 

 nuclei closer. 



The supposed mode by which growth through 

 intussusception is brought about was first de- 

 scribed by Nagelli. He supposes that the cellulose 

 layer of a starch granule is composed of isolated 

 solid molecules of various sizes held together by 

 the force of cohesion and mutual attraction, be- 

 tween which water penetrates — the molecules them- 

 selves, as we might expect, being quite invisible 

 even with the highest powers of the microscope. 

 The gradual increase in size of the granule is thus 

 due to the absorption of water and formative mate- 

 rial to the nucleus when it undergoes chemical 



transformation. Then its formation into granular 

 and new cellular layers causes a swelling of the 

 external cellulose by the pushing further apart of 

 its molecules. If contraction take place, it is 

 through the withdrawal of water from the inter- 

 stices. 



Fig. 120. Potato Starch {polarized). 



We will now pass on to the more practical ex- 

 amination of starch granules. Probably every 

 beginner at the use of the microscope has examined 

 a thin section of a potato, than which nothing can 

 better show the various sizes, shape, and position of 

 starch granules, as they appear in their natural 

 position in the cell. 



(cy, 



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Fig. 121. Starch Grams of Natal Arrowroot, Bermuda Arrow- 

 root, and Wheat (latter concentric lamination). 



The eccentric lamination is with a little care, as - 

 to light and focussing, easily seen and examined, as 

 the granules roll about and over one another if a 

 drop of water be placed on the slide. Now, though 

 it would be difficult, and perhaps impossible, to> 

 state with certainty that any given granule was- 

 potato starch and no other, yet, examined collec- 

 tively, and with regard to measurement of size, it 

 would be impossible to mistake it for any other. 



You might pick out, say half a dozen granules of 

 Tous-les-Mois starch, and half a dozen of potato, 

 and perhaps the difference would be indistinguish- 

 able; but examine any samples of each separately, 

 and Tous-les-Mois will be found to be uniformly of 

 a larger size, irrespective of other differences. 



The importance of this branch of the subject in- 



