640 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tissues wliich will swell sufficiently under its action, the chlor-zinc- 

 iod method may be regarded as perfectly satisfactory; after treat- 

 ment with picric-Hoffmann' s-blue and subsequent washing with water, 

 nothing but protoplasmic structures will be stained. In clear instances 

 where a thick closing membrane is plainly traversed by threads, it can 

 be demonstrated with ease that, while the individual threads are well 

 stained, the substance of the pit-membrane itself undergoes no colora- 

 tion, even when the section has been exposed to the action of the dye 

 for a long time. When the pits are smaller and the threads less 

 clearly defined, it is more difficult to observe that the substance of the 

 pit-membrane is still free from coloration ; and when, owing to the 

 thinness of the closing membrane, all appearances even of striation 

 cease to be recognizable, only an apparent staining of the entire 

 membrane can be observed. Such staining points, however, in the 

 opinion of the author, not to the coloration of the substance of the 

 pit-membrane, but to the staining of protoplasmic threads traversing 

 its structure. 



Besides a platinum lifter, the author uses platinum needles, and is 

 careful thoroughly to brush all the sections with a camel's-hair brush, 

 both after the action of the acid or of chlor-zinc-iod and after 

 staining. 



To prove that the threads traversing the cell- wall are actually pro- 

 toplasm, he employed with success a solution of molybdic acid in strong 

 sulphuric acid, which has the advantage of swelling the cell- wall and 

 at the same time colouring the protoplasm. The solution is colourless 

 and gives a beautiful blue colour with alcohol and many other organic 

 substances ; and this reaction is extremely delicate. While not 

 affecting the cell-wall for some time this reagent gives at once a fine 

 blue coloration with protoplasm. If a section of some living endo- 

 sperm, such as that of Tamus, is treated with it, the cell-wall w^ill 

 swell up, and it will commence to dissolve the protoplasm ; the fine 

 threads perforating the walls will remain for some time unaffected, 

 but will soon be perceptibly coloured, while the main mass of proto- 

 plasm will assume an intense blue. 



The pit-membrane itself possesses some properties different from 

 those of the cell-wall. After staining with iodine and chlor-zinc- 

 iod, while the cell-wall assumes the usual blue tint, the pit-mem- 

 brane is but slightly coloured, and, when thin, appears as if not coloured 

 at all, although the examination of a fine transverse section of the pit 

 will prove that a definite staining has taken place. But the depth of 

 the staining is less than might have been expected in proportion to the 

 thickness of the membrane. Methylene blue stains both the wall and 

 the pit-membranes a fine light blue, and, after the action of sulphuric 

 acid, the swollen wall assumes a much lighter tint, owing to the fact 

 that the quantity of the dye taken up by the cell-wall is now distri- 

 buted over a larger space. If a section is cautiously treated with 

 sulphuric acid, washed, and stained, it will be seen that, whereas the 

 general swollen wall is coloured a light blue, the bottoms and the sides 

 of the pit retain the darker blue colour of the unswollen cell-wall, and 



