392 



BOTANY 



PAllT II 



In some Pennatae the sexuality is lost and the auxospores arise apogamously. Jn 

 Synedra the mother cell divides into two cells which grow into auxospores ; the 



nuclei of the daughter cells 

 L j, 



undergo a division, but the 

 resulting nuclei again fuse. 

 In Rhabdonema arcuatum 

 the process is similar, but 

 the second nuclear division 

 does not occur. 



Rhabdonema adriaticum 

 goes a step farther ; the 

 nucleus divides, but one of 

 the daughter nuclei is ex- 

 truded from the protoplast. 

 The undivided mother cell 

 develops into the auxospore. 

 Many Pennatae occur in 

 places where decomposing 

 substances are present in 

 abundance. 'Such species 

 can assume a saprophytic 

 mode of life, their chromato- 

 phores becoming colourless 

 and reduced in size. It has 



1, Vegetative cell; , pair of been shownthatsomecolour- 

 less species of Nitzschia 

 which occur in the sea are 

 exclusively dependent on 

 organic substances for food, the reduction of their chromatophores and pigment 

 being complete (^}. 



Navicula ostrearia is a Diatom occurring in the oyster-beds of the French coast, 

 which serves as food for the oysters ; its protoplasm contains a sky-blue pigment 

 called marennin. This pigment is the cause of a green coloration of the oysters 

 in which the marennin undergoes alteration and is accumulated ( 23 ). 



Diatoms occur also as fossils. Their silicified valves form a large part of the 

 deposits of SILICEOUS EARTH (Kieselguhr, mountain meal, etc.), and in this form 

 they are utilised in the manufacture of dynamite. 



On account of the extreme fineness of the markings of their valves, it is 

 customary to employ certain species of Diatoms as test objects for trying the 

 lenses of microscopes. Pleurosigma angulatum is commonly used for this purpose. 



FIG. 324. Coccone/is placentula. 



cells before conjugation ; 3, k, cells in process of conjuga 

 tion. gk, Large nucleus ; kk, small nucleus ; g, gelatinous 

 substance. (After KARSTUN, from OLTMANNS' Algae.) 



CLASS VII 

 Conjugatae ( T > u > 24 ' 26 ) 



This class of green fresh-water Algae includes unicellular and 

 simply filamentous forms, and is clearly distinct from that of the 

 Chlorophyceae. Their cells, which increase in number by cell division, 

 are uninuclear, and differ from those of the Diatoms in having a cell 

 wall which is not silicified, and in the presence of large green chloro- 



