THE PROTOPLAST 



17 



In commencing the study of cells an excellent object for microscopic 

 examination is the thin skin which covers the inside of the fleshy scales 

 of the common onion. The object depicted in fig. 35 consists of a small 

 fragment of this delicate membrane, mounted in balsam. Observe that 

 it is made up of a number of hexagonal or six-sided figures, on the 

 interior of which is an irregular granular substance. Each of these 

 hexagons represents a perfect cell; its sides are cell-walls, and the granular 

 matter in each is protoplasm. 



Protoplasm in its natural 

 state is colourless and trans- 

 parent so transparent as only to 

 be distinguished with difficulty 

 under the microscope. To make 

 it more apparent the specimen is 

 soaked in iodine solution, which 

 has the effect of staining living 

 protoplasm brown, while it tinges 

 with pale yellow the lifeless walls 

 of cellulose. The pale yellow is 

 hardly noticeable by lamp-light ; 

 but if a drop of strong sulphuric 

 acid is run under the cover-glass 

 at the time of preparing the slide, 

 the cell-walls become coloured 

 blue. It is by the use of these and 

 other reagents that the organic 

 elements of cells and tissues are 

 distinguished. The word " proto- 

 plasm " appears to have been first 

 used by Purkinje* in 1840, to 

 denote the formative substance of 

 the animal embryo, which he com- 

 pared with the soft cellular tissue 

 (cambium) between the wood and 

 the barkof trees. Mohl,afew years 

 later (1846), applied the term to 

 the contents of the vegetable cell. 



It will be noticed that the protoplasm of each of the onion cells contains 

 a small spherical or oval mass, which takes a darker brown than the sur- 

 rounding matter when treated with iodine. The darker colour is due to 

 the greater density of the protoplasm at these points ; and these denser 

 portions envelope a sort of kernel the nucleus (Lat. nux, a nut or kernel), 



* The substance itself was first noticed and described by Roesel v. Rosenhof in his account 

 of the Proteus-animalcule, and was named sarcoda by Dujardin in 1835. 

 4 



FIG. 34. BEADED HAIRS OF VIRGINIAN 

 SPIDERWORT (Tradescantia). 



Showing the rotation of the protoplasm 



swollen part in the stream of protoplas 



Greatly magnified. 



their cells. Each 

 denotes a nucleus. 



