1(56 THE CELL 



ternal wall of the capsule, with its opening, becomes differentiated, 

 and around it the contractile sheath develops. 



I. External Plasmic Products. The external plasmic pro- 

 ducts may be divided into three groups, cell membranes, cuticular 

 formations, and intercellular substances. 



Cell membranes are structures which separate out, and envelop 

 the whole surface of the cell-body. In the vegetable kingdom 

 they are very important, and easily seen, whilst in the animal 

 kingdom they are frequently absent, or are so slightly developed 

 that they can hardly be made out even with the strongest powers 

 of the microscope. 



In plants, the cell membrane is composed of cellulose, a carbo- 

 hydrate very nearly allied to starch. The presence of this sub- 

 stance may generally be easily demonstrated by a very character- 

 istic reaction. If a section of a plant tissue, or a single plant cell, 

 is saturated first with a dilute solution of iodine in potassic iodide, 

 and then (after the excess of the iodine solution has been removed) 

 the preparation is immersed in sulphuric acid (2 parts acid to 

 1 part water), the cell membranes assume a lighter or darker blue 

 coloration. Another reaction for cellulose is seen when chlorzinc- 

 iodine solution is used (Schulze's solution). 



The membranes of plant cells often become thick and firm, and 

 then they show, in section, a distinctly marked striation, being 

 composed, like starch grains, of alternate bands of high and low 



FIG. 71. FIQ. 72. 



FIG. 71. Transverse section through the thallus of Caulerpa prolifera at the place where 

 a branch is inserted. (After Strasburger, PI. I.. Fig. 1.) 



FIG. 72. A Portion of a fairly old pith cell, with six layers from Clematis viialba (after 

 Strasburger, PI. I., Fig. 13); B a similar cell after it has been swollen up by sulphuric 

 acid. (After Strasburger, PI. I., Fig. 14.) 



