82 Practical Plant Biology. 



which the somatic cells are differentiated they are cut off from this 

 immortality. Hence with the differentiation of an individual into 

 germ cells and somatic cells mortality is introduced into the 

 species ; and as this differentiation is only possible in multicellular 

 organisms we find that mortality is introduced with the transition 

 from the unicellular to the multicellular state. 



From this point of view we may regard the germ cells of any 

 multicellular organism as continuous lines of living protoplasm 

 producing at regular intervals excrescences by cell-division. The 

 individuals are these excrescences developed periodically on 

 the lines which, it may be, extend through geological epochs. 

 In unicellular organisms these periodic excrescences are not 

 developed. 



PRACTICAL WORK. 



Examine spheres of Volvox in a drop of water uncovered. Observe the 

 direction of the motion of the free-swimming individuals Reverse the slide 

 on the stage so that the side which was nearest the window is now furthest. 

 Again observe the motion. Cut off the direct light and observe the organism 

 by means of the light transmitted from the mirror through the condenser 

 alone. 



Before applying the high power a cover-glass must be put on. To prevent 

 the latter crushing the Volvox put a few fibres of cotton-wool in the drop 

 before laying on the cover-glass. With the high power focussed on the side of 

 the sphere turned towards the observer, it should be possible to make out the 

 individual cells as seen from in front, also their protoplasmic connections, the 

 chloroplast, pyrenoid and nucleus. If we are observing the anterior pole, 

 stigmata or eye-spots should be seen in each of the cells. It often requires 

 very careful focussing and adjustment of the light before these structures can 

 be made out ; sometimes by focussing on the outer surface of the sphere the 

 cilia may be seen. 



The same structures should be looked for in the limb or profile of the 

 sphere. From this aspect we see a side-view of the cells. The colourless 

 beaks coming to the surface of the sphere should be noted and the cilia 

 projecting from them looked for. Also this profile-view shows the chloro- 

 plast, pyrenoid and nucleus more easily than the previous one. The stigma 

 may also be found, but the protoplasmic connections are difficult to make 

 out. 



A sphere mounted in liquor iodi shows the cilia and protoplasmic connec- 

 tions plainly. 



To observe the pellicle which coats the outer and radial cell-walls spheres 

 fixed in 4 per cent, formalin and stained with watery gentian violet are 

 suitable. 



