CHAPTER III. 



THE CELL-WALL. 



23. In all but the lowest plants the protoplasm of every 

 cell surrounds itself sooner or later with a covering or wall 

 of cellulose. The substance of the cell- wall is a secretion 

 from the protoplasm. Cellulose, as such, does not exist in 

 the protoplasm ; it is formed on the surface when the wall is 

 made. On its first appearance the wall is an extremely thin 

 membrane, but by subsequent additions it may acquire vary- 

 ing degrees of thickness. The cell-wall forms a complete 

 covering for the protoplasm ; there are at first no openings 

 in it, at least none that are visible ; later in the life of the 

 cell pores are formed in the wall in some cases, while quite 

 frequently in dead cell-walls there are large perforations of 

 various sizes and shapes. 



(a) Cellulose is related chemically to starch and sugar. Its composi- 

 tion is On H 80 Oio. It is tough and elastic. It is but slightly soluble 

 in dilute acids and alkalies, and not at all in water and alcohol. In 

 water, however, it swells up from imbibing some of the liquid, but it 

 shrinks again in bulk when dried. 



(b) Tests. 1. If cellulose is treated with dilute sulphuric acid, and 

 shortly afterward with a weak solution of iodine, it is colored blue. 



2. Treated with Scliultz's Solution it assumes a blue color. 



(c) In the Myxomycetes, if the large mass of protoplasm composing a 

 plant is somewhat dried, it gepnrates itself into smaller masses, which 

 surround themselves with a cell-wall. Upon applying sulphuric acid 

 and iodine, the characteristic blue color of cellulose appears, showing 

 that the wall is a true wall of cellulose. If, however, any such dried 

 mass of protoplasm is subjected to the proper conditions of moisture 

 and temperature, the cell-wall is dissolved and absorbed into the proto- 

 plasmic mass. Tests applied now utterly fail to show the presence of 

 cellulose. These observations prove the truth of the statement that 

 cellulose is a secretion, and that it is not contained, as cellulose, in the 

 protoplasm. 



