200 H. von Mohl on the Investigation of Vegetable Tissue 



lengthwise, and in which black longitudinal stripe are visible in 

 polarized light. He was inclined to attribute the doubly-re- 

 fracting property of starch-granules and the substance contained 

 in the scales of plants, not to conditions of tension, but to a 

 minute crystalline condition, somewhat comparable to the con- 

 centric arrangement of the needles of carbonate of lime in pearls 

 and pisolites. 



Schacht's essay is an extremely small affair. It acquaints us 

 with one single correctly observed fact not mentioned by his pre- 

 decessors, namely the appearance of a black cross in the vicinity 

 of the pits of fir-wood, of the endosperm-cells of Phytelephas, &c. 

 All the rest of his statements (omitting the long-known and un- 

 mistakeable fact that starch-granules and ti-ansverse sections of 

 thick-walled cells of plants exhibit a black cross) are false. The 

 whole work is consequently not merely valueless in itself, but 

 positively injurious, since it is only calculated to lead inexperi- 

 enced readers into error. According to Schacht, unstratiried 

 membranes (cambium-cells, the parenchyma of young organs) 

 do not possess the power of doubly- refracting light. This is 

 totally false : equally devoid of truth is the statement that the 

 polarizing microscope may be employed to detect whether vege- 

 table cell-membrane has formed broad layers of thickening. Not 

 less false is the further assertion, that the chemical character of 

 vegetable matter is not concerned in its effects upon polarized 

 light : the very reverse is the case. In the same way, the asser- 

 tion is incorrect that vegetable membranes do not act upon 

 polarized light when it passes through in a direction perpen- 

 dicular to their surface. 



If my investigations have led to the discovery of phsenomena 

 that escaped my predecessors, the reason lies not in the application 

 of new apparatus, but chiefly in the circumstance that I took 

 great pains to give the polarizing microscope the most advan- 

 tageous arrangement, and especially to improve the illumination. 

 It would lead me too far to enter into a description of the con- 

 trivances that suggested themselves to me ; I shall take another 

 opportunity of doing this. 



In passing to the account of the phsenomena which the ve- 

 getable tissues exhibit in the polarizing microscope, I shall 

 confine myself to the detail of the facts, and only intersperse 

 theoretical considerations so far as they may appear necessary for 

 the comprehension of the phsenomena, by those who are not 

 acquainted with the properties of polarized light and of doubly- 

 refracting substances, which come into play here. 



I commence with the consideration of the cellulose membranes. 

 The action which these exert upon polarized light, is perhaps 

 shown most clearly in the examination of a thin transverse sec- 



