H 



THALLOPHYTA 



known as infusorial earth or kieselgllhr. Some of these are of 

 economic importance, being used in the manufacture of dynamite, ami tor 



other purposes. All diatoms are unicellular, though some are united by the 

 mucilage they secrete into colonies or filaments. They occur both in fresh 

 and salt water, and are more abundant in the colder parts of the world. 

 1 [ence diatoms, although common, are not so frequent in South Africa 

 as they are in colder countries. 



The structure of a diatom is somewhat remarkable. Each individual is 

 known as a frustule. The cell-wall consists essentially of two valves 

 which fit closely together, like a pill box and its lid. In the commonest 

 genus of diatoms, Navicula^ each valve has the shape of a boat, one, turned 

 upside down, fitting closely into the other, there being just sufficient difference 

 in size between the two to make this possible. Further, that part of each 

 valve which overlaps the other seems to have a different structure from, and 

 to be rather attached to than continuous with, the rest ot the valve. This 

 part is known as the girdle. 



..#. 



Fi< 



9. Navicula viridis (Nitzsch) Kiltz. 

 1. Face-view. 2. Lateral view (girdle). 



1 000/ 1. (After Pfitzer) 



10. Formation of auxospores of 

 Navicula viridula Kiltz. A. Face-view 

 of cell. B. Two cells, their contents 

 dividing each into two daughter-cells. 

 C, D. Copulation of corresponding 



couples, each one forming one auxo- 



spore with 4 nuclei. E. The two adult 



auxospores, 700/1. (After Karsten) 



It follows from what has been said that there are three points of view 

 from which a diatom may be examined : 



1. Looking down on one of the valves, known as the valve view 



(Fig- 9, 0- 



II. Looking down on the girdle {i.e. with the two valves in profile) 



known as the girdle view (Fig. 9, 2). 

 III. Looking at the end of the diatom, known as the end view. 



