THE CELL-WALL. 491 



which different views are entertained. On the one hand it is said that 

 the cellulose produced in the formation of the original membrane or layer 

 of thickening becomes gradually converted into different but related 

 chemical compounds ; on the other, that the cellulose layers become im- 

 pregnated by foreign substances, gradually infused into them from the 

 fluid contents, sucli substances being distinguished by the name of in- 

 crusting matters (fig. 543). A third view is that of Fremy, who con- 

 siders that there are several kinds or modifications of cellulose, and, more- 

 over, that those vegetable structures formerly considered to be made 

 up exclusively of cellulose contain matters of a different chemical com- 

 position. 



Action of Reagents on Cellulose. Cellulose, as found in the organized 

 condition of cell-membrane, appears to behave somewhat differently to 

 chemical reagents according to the state of aggregation of its particles 

 (that is to say, its density) ; for nascent cell-membranes will in many 

 cases assume a violet or even a blue colour when treated with a strong 

 solution of iodine and washed with water, like starch. The same is the 

 case with some of the sernigelatinous layers of thickening met with in the 

 endosperm or cotyledons of certain seeds (called amyloid}, and, moreover, 

 in the cell-structures generally which have been treated in the way de- 

 scribed below, to remove the so-called " incrusting matters." But, as a 

 general rule, cellulose does not take a blue colour with aqueous solution 

 of iodine, unless some other agent, especially sulphuric acid, be applied at 

 the same time. A solution of iodine in zinc chloride brings out a blue 

 colour in fully developed cell-membranes, still more readily than the 

 sulphuric acid with iodine. These reagents readily affect newly formed 

 tissues in general ; and the more delicate kinds of cellular tissues are 

 permanently sensitive to them. But after a time the thicker cell- 

 membranes, and especially those of woody tissues, the cartilaginous 

 structures, and the tissue of epidermis and bark, no longer become blue, 

 but only yellow or brown, with the above reagents ; and it is the real 

 cause of this alteration which is the subject of the difference of opinion 

 above referred to. Anilin and sulphuric acid stain the lignified cells 

 yellow *. 



By maceration for several hours, or boiling for a minute or two, in nitric 

 acid for woody and cartilaginous tissues or in strong solution of potash for 

 epidermal and corky tissues, bringing the cells to a point where they still 

 exhibit all their structure, but are bleached and softened, then washing 



* The iodide solution consists of 1 grain of iodine, 3 grains of potassic 

 iodide, and 1 ounce of distilled water. The sulphuric acid is of the strength 

 of three parts acid to one of water. The preparation to be examined is first 

 moistened with a drop of the iodine solution, which is then wiped off with a 

 brush or piece of blotting-paper, and a drop of the acid is then placed on the 

 preparation, which is covered with thin glass in the usual way. Schulze's solu- 

 tion is made by dissolving an excess of metallic zinc in strong hydrochloric 

 acid, allowing the solution to evaporate over a spirit-lamp to the consistence 

 of syrup, the zinc being kept constantly added if necessary. To the now 

 colourless syrup is added as much potassic iodide as it will take up. The ad- 

 vantage of Schulze's solution is that it does not destroy the tissues, acts more 

 speedily, and is less injurious to the microscope should it happen to come into 

 contact with it. 



