VII 



THE HISTOLOGY OF THE CELL 81 



near the centre of the field and proceed to observe a single cell with the 

 high power. Make out 



(i) The cell-wall, protoplasm, nucleus, and vacuoles> 



(ii) Treat with iodine solution ; the protoplasm is stained brown, and 

 the nucleus a very dark brown. 



(iii) Treat a freshly prepared specimen with salt solution (2\ per cent. ). 

 The wall retains its original position and appearance, but the protoplasm 

 contracts and leaves the walls. This is known as plasjnolysis. 



(iv) Wash out the salt solution with water and 'exainlne again ; the 

 protoplasm slowly regains its original position. 



The contraction of the protoplasm is due to the salt solution 

 attracting the water from the cell ; and it regains its original 

 position when water again is taken in. 



The Nucleus. The nucleus is a denser portion of the proto- 

 plasm ; it stains a deeper colour when treated with iodine 

 solution. In shape the nucleus is somewhat oval, and in its 

 interior a distinct rounded body called a nucleohts may be pre^ 

 sent. It is built up of proteids, and contains a large quantity of 

 phosphorus. A nucleus is present in all cells, and this seems to 

 show that the presence of such a body is necessary to the life of 

 the cell. It is always formed from a preceding nucleus. The 

 exact function of the nucleus is not known, but in every case of 

 cell-production the nucleus divides first. It has been suggested 

 that the nucleus is the most important part of the cell, and that 

 it forms the protoplasm which surrounds it. 



The Difference between a Young and Mature 

 Cell. A very young cell is completely filled with protoplasm. 

 As the cell increases in size the cell-wall grows faster than the 

 protoplasm, causing cavities, which become filled with cell-sap, 

 to appear in it. These cavities are called vacuoles, which in a 

 very old cell may be very large. 



The Contents of the Cell. The cell always contains a 

 number of other substances in addition to the protoplasm and 

 the nucleus. In fact, at one time or another, it contains every 

 element that the plant contains, for the puatoplasm is the active 

 material of the cell, and produces all the organic substances 

 found in the plant. The vacuoles and all parts of the cell are 

 saturated with cell-sap. The protoplasm contains granules, 

 which, according to their nature, are variously known as chloro- 

 plasts, leucoplasts, and chromoplasts. Starch and Aleurone 

 grains are also found in cells, while fats and, in some cases, 

 crystals of calcium oxalate may be present. 



G 



