62 OUTLINES OF EVOLUTIONABY BIOLOGY 



The granular, colourless protoplasm does not fill the interior 

 of the cell in a uniform manner, but is arranged partly as a 

 thin lining to the cell-wall, known as the primordial utricle 

 (p.n.), and partly in irregular strings which branch and anas- 

 tomose and stretch across the cavity of the cell in various 

 directions. These strings of protoplasm tend to converge towards 

 an irregular mass in which the nucleus is situated. The nucleus 

 itself (nu.) is a nearly spherical body of denser protoplasm, about 

 0'024 mm. in diameter. The extensive space which lies inside 

 the primordial utricle and between the strands of protoplasm 

 is filled with a more fluid liquid known as the cell-sap. It is 

 to this cell-sap that the flowers of Tradescantia owe their 

 colour; if the flowers are blue the cell-sap will be found to 

 be blue and if they are white it is because the cell-sap is 

 colourless. 



The most striking feature of the cell which we are examining 

 still remains to be noticed. The protoplasm is in constant move- 

 ment. This is at once evident from the characteristic streaming 

 of the small granules which it contains. Both in the primordial 

 utricle and in the network of threads a constant circulation is 

 kept up, though not in a very definite manner, as the threads 

 themselves are constantly undergoing slow changes in their 

 arrangement. The arrows in Fig. 25, B indicate approximately 

 the course taken by the streaming protoplasm at the time when 

 the drawing was made. 



The streaming of the protoplasm appears at first sight to be 

 an essentially vital phenomenon, but it is probably merely ther 

 mechanical result of chemical and physical processes going on 

 in the cell, such as the diffusion of various substances in solution 

 from one cell to another, which must take place in the process of 

 nutrition. If the cell is killed by the addition of alcohol the 

 physical and chemical conditions are at once altered and the 

 movement ceases ; the protoplasm is coagulated and, if we are 

 dealing with a cell containing coloured cell-sap, the nucleus 

 absorbs the colouring matter with great avidity and becomes 

 deeply stained, while the cytoplasm stains only very slightly or 

 not at all. We are thus able to make the cell stain itself 

 differentially, without the aid of any extraneous colouring matter, 

 the cell-sap acting as what is termed a nuclear stain. 



It will be necessary to restrict our further observations on 

 Tradescantia to the structure of the leaf (Fig. 26). The leaves 



