DIATOM ACEAE 15 



On looking at a number of diatoms under the microscope, it generally 

 happens that practically all are lying so as to exhibit either the " valve " or 

 the "girdle" view, whereas it is a more difficult matter to see them in "end 

 view." The two valves are exactly alike, except for a trifling difference in 

 size, so the frustule is symmetrical about three planes at right angles to one 

 another. The valves are often ornamented with numerous fine transverse 

 markings, and in many cases a longitudinal opening (visible as a line) is 

 found running nearly the whole length of each valve. This is known as 

 the vaphe. Immediately within this are found three swollen ftodtlles, 

 one in the centre, the central nodule, and one at each end of the raphe, 

 the terminal nodules. Diatoms possessing a raphe exhibit peculiar movements, 

 the exact cause of which is not fully understood, though it appears to be 

 connected with the extrusion of mucilage from the nodules, through which 

 run very fine pores called cwnalictlli. Each frustule has a central nucleus, 

 and one or more chloroplasts embedded in its lining layer of protoplasm, the 

 green colour of which is however quite masked by the golden brown colouring 

 matter, diatomi'Vl (Plate 2, Fig. F), which is always present. 



Reproduction occurs (1) by division of the cells, and (2) by the formation 

 of special spores called awxospoves. In the first named method a swelling 

 of the contents of a frustule pushes the valves apart. The contents then 

 divide into two parts, each part retaining one valve of the original frustule. 

 Then each daughter- cell forms a new valve to fit inside the old one. Since 

 the valves are incapable of growth in most cases, one of the daughter-cells 

 formed at each division will be smaller than the mother-cell. This reduction 

 in size of the frustules does not, however, proceed indefinitely, but when 

 a certain minimum size has been attained, auxospore formation restores the 

 original size. 



As the name implies the auxospore is a much larger body than the cells 

 from which it is formed, rapid growth taking place during its formation. It 

 may be formed in various ways, either sexually or non-sexually. The method 

 in Navicula may serve as an example of one of the more complex ways 

 (Fig. 10). In this case two frustules come to lie side by side, the contents 

 of each swell, cast off the valves and divide, and the nucleus of each daughter- 

 cell divides again to form one large and one smaller nucleus ; then each 

 daughter-cell fuses with one of the daughter-cells of the other frustule, the 

 larger nuclei uniting, the smaller disintegrating : each of the two resulting 

 cells is an auxospore. Gradually the auxospore assumes the form of the 

 ordinary frustule. 



IX. PHAEOPHYCEAE. 



The Brown Algae. 



The Phaeophyceae include some of the largest known plants, the length 

 of the largest species being three times that of the tallest forest trees. They 

 range in size from these marine giants to forms scarcely visible to the 

 naked eye. 



