SPECIAL METHODS. 1 67 



her of dyes which leave pectic substances uncolored, but 

 stain cellulose or both cellulose and callose. To the former 

 belong orseille red A, naphtol black, and the croceins ; while 

 Congo-red, azo-blue, and benzopurpurin stain cellulose and 

 callose. 



294. In order to show that the first described stainings 

 really depend upon the presence of pectic substances, Man- 

 gin treated thin sections for 24 hours with cuprammonia 

 and then washed them in water and in 2% acetic acid. On 

 this treatment the cellulose is removed from the membranes 

 and fills the intercellular spaces and the cell-cavities as a 

 gelatinous mass. In consequence of it the membranes are 

 colored not at all or but slightly yellow on the addition of 

 chloroiodide of zinc, while a deep blue color appears in the 

 interiors of the cells. The membranes, which now consist 

 of pure pectic acid, are, however, deeply stained by safranin 

 or methylene blue. It is sufficient to add a few drops of 

 a solution of ammonium oxalate to cause the solution of 

 the pectic acid membranes. 



295. In order to show that the middle lamella of the 

 so-called cellulose membranes consists of pectic acid or an 

 insoluble salt of it, Mangin (VI) lets a mixture of one part 

 hydrochloric acid and 4 to 5 parts alcohol act for 24 hours 

 on thin sections, then washes them with water, and treats 

 them with a weak (about 10$) solution of ammonia. After 

 this has acted a short time, the sections may be separated 

 into their constituent cells by gentle pressure. Mangin 

 explains this by the supposition that the pectic acid is set 

 free from its originally insoluble compounds by the action 

 of the acid-alcohol, and is then dissolved by the ammonia 

 solution. In fact, a gelatinous mass is precipitated from the 

 ammoniacal solution on the addition of acid, which has all 

 the characters of pectic acid. On the other hand, sections 

 which were placed in lime- or baryta-water after the action 

 of the acid-alcohol, showed no separation into their cells on 

 subsequent treatment with ammonia, because the pectic 

 acid had recombined into an insoluble salt with the alkaline 

 earth. 



