ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



357 



The manufacturers of oleomargarine, however, made further 

 improvements, and it was so free from crystals of fat that the Nicols 

 failed to distinguish them from butter. He therefore introduced a 

 selenite plate, the object of which was to detect fatty bodies in a 

 homogeneous state. Although not so much as a single crystalline 

 form may be present, all the prismatic colours are shown throughout 

 the homogeneous mass, while pure butter exhibits under the same 

 conditions only plain red or green. A non-microscopic test is also 

 given by the author. A coloured plate illustrates the paper. 



Polarized Light in Vegetable Histology.* — L. Dippel directs 

 attention to a method of observation by polarized light which has 

 afforded him much assistance in researches into the minute structure 

 of the cell-wall. 



Examined with ordinary light, a very thin transverse section of a 

 tissue with thickened cell-walls, cut perpendicularly to its long axis, 

 exhibits the so-called " middle-layer " of Hofmeister, Sachs, and others, 

 in which, beyond the well-known gusset in the angles, no further 

 differentiation is perceptible (fig. 94). Under polarized light, how- 

 ever, and with crossed Nicols, there is a substantial alteration in its 

 appearance (fig. 95). The apparently homogeneous structure is 

 traversed by a fine black line, and is thus divided into three parts. 



Fig. 94. 



Fig. 95. 



Observation with polarized light thus tells us very decisively that 

 the " middle-layer " is not simple, but consists of three laminse, of 

 which the central one is singly refracting, and the two lateral ones 

 doubly refracting. The former is therefore of a different molecular 

 structure, and even chemical constitution, from the other two. 

 Polarized light may also be appealed to to support the results 

 obtained by maceration and reagents, which have been questioned. 



On the question of the nature of the first solid secretion-product 

 of the living cell-body, i. e. the first wall-formation to which the 

 "middle-layer" owes its origin, observation with polarized light 

 also affords an explanation if applied to the cell- wall during its 



* Zeitschr. f. Wiss. Mikr., i. (1884) pp. 210-7 (5 figs.). Transl. in Mier. 

 News, iv. (1884) pp. 291-7 (5 figs.). 



