274 H. von Mohl on the Investigation of Vegetable Tissue 



circular curves, to which is added, in the pits of Fir-wood, the 

 circumstance that the cell-wall is protruded inwards into the 

 cell, in a globular form, over the cavity which forms the border 

 of the pit. 



Passing from the cell-wall to the cell-contents, we find that 

 the behaviour of starch- granules with polarized light has already 

 been investigated by so many persons, that scarcely any notice 

 will be required respecting it. It is universally known that they 

 show a black cross, like the transverse section of a cylindrical 

 cell ; that the point from which the arms of the cross run out 

 always coincides with the organic centre of the granule, with 

 the so-called hilum ; that the arms of the cross stand perpendi- 

 cular to the lamellse of the granule, and that therefore the cross 

 is often exceedingly irregular*, from the excentric arrangement 

 so frequently presented by the lamellse (most strikingly in the 

 starch -granules of the tubers of Canna indica, of Galanga-root, 

 of the milky juice of Euphorbifs, &c.). I have likewise explained 

 above, that the colours which the starch- granules exhibit on the 

 application of a plate of selenite, are positive, and opposite to 

 those of cellulose. The swelling-up of starch -granules in boiling 

 water, strong acids, or caustic alkalies, removes from their sub- 

 stance the power of acting upon polarized light (at least in a 

 degree capable of detection with our instruments). It offers a 

 very attractive spectacle to observe this process in the starch of 

 the Potato with the application of a selenite-plate. When a drop 

 of solution of potash is added to the water in which the granules 

 lie, the latter swell from without inwards : as far as their sub- 

 stance remains yet unattacked, it exhibits the most vivid colours ; 

 the swollen portion is quite sharply separated from the still hard 

 parts, and during its expansion it exhibits pale colours, which 

 vanish in the completely gelatinized parts. 



In my opinion, inulin does not occur in the form of granules 

 in the cells of living plants, but in solution. In the parenchyma- 

 cells of dried roots oi Inula Helenium, it presented itself in the form 

 of irregular angular masses, not expanding in cold water, acting 

 strongly upon polarized light, which possibly may depend simply 

 upon mechanical tension. It was precipitated from the boiling 

 solution in the form of small, irregular, roundish lumps, which 

 acted only weakly and in an irregular manner upon polarized light. 



I could not observe any effect upon polarized light in chloro- 

 phyll-granules or the chlorophyll-bands of Spirogyra. The 

 starch-granules contained in them displayed the ordinary phse- 



* That particular observers, for example Pereira, did not see the black 

 cross in the starch-granules of certain plants, for instance in the seeds of 

 wheat and rice, is attributable to the imperfection of the instruments with 

 which they observed. 



