THE CELL 41 



conspicuous by a great number of violet hairs (fig. 15, 

 B), each of which consists of round or oval cells, 

 arranged in a row, like a rosary. If you detach one 

 of these threads with a needle and place it under the 

 microscope you will notice younger cells at the tip of 

 it which are nearly round, whereas at the bottom the 

 cells are older and oblong (fig. 15, C). 



To begin with, we distinguish in such a cell between 

 its thin and perfectly transparent wall and the actual 

 contents of the cell. At first the cavity of the cell is 

 filled by a uniform, semi-fluid mass called protoplasm, 

 with a round body called a nucleus embedded in it, which 

 we shall study later on. Subsequently little spots 

 appear in the semi-fluid protoplasm, like cheese eyes, 

 so to speak, filled with liquid. Thus the contents of 

 the cell become separated into two parts, the proto- 

 plasm and the liquid cell-sap, becoming more and more 

 frothy. Later still the proportion of sap to protoplasm 

 increases ; the volume of the protoplasm diminishes 

 relatively as that of the cell augments. In the end 

 almost the entire cavity of the cell becomes filled with 

 the watery sap, and the protoplasm remains only as a 

 thin layer, lining the inner wall of the cell, or stretching 

 from one wall to the other in little strands. In Trades- 

 cantia such a differentiation of the contents of the cell 

 is particularly well marked, because the cell-sap is violet 

 in colour while the protoplasm is colourless. Besides 

 these two substances, protoplasm and cell-sap, we also 

 frequently notice in the cavity of the cell something of 

 a different kind — small, shining drops with an oily 

 appearance, or round, colourless little grains, the char- 

 acteristics of which will be studied later. At a later 

 stage the contents of the cell sometimes disappear, and 

 the cavity fills with air. Such a skeleton of a cell must 

 be considered dead. The dry, sapless part of a tree, 

 for instance, may be considered as formed of such dead 

 cells. Thus in a living, active cell the microscope 



