354 



BOTANY 



PAET II 



silicified membrane (perizoiiium), and in this the new valves are formed (Fig. 

 269, £). 



The Centricae i'urther differ from the Pennatae by possessing a special method 

 of reproduction by means of so-called microspores ; the formation of these has been 

 accurately followed by Bergen in BiddidpJiia mohilknsis. A cell first divides into 

 two daughter cells or sporangia, the contents of whicli round off and by repeated 

 division form many (32) microspores. These emerge as naked swarm-spores, each 

 provided with two long cilia with knob-like thickenings at the tip (Fig. 269, 

 C-E). These swarm-spores appear to behave as gametes, for Kakstex observed in 

 preserved material of Corethron valdiviae that the microspores conjugated in pairs 

 to form zygotes. The zygote increased in size and divided into two cells. Each 

 daughter cell had at hrst two nuclei, one of which later disappeared ; it grew 

 gradually into a mature individual of Corethron. The whole process may be 

 compared with that described above in Closteriwni among the Desmidiaceae (-■'). 

 The ciliated gametes or microspores indicate a phylogenetic relation of the 

 Diatomeae to the Peridineae and Flagellatae. 



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. Flcurosigma angidatum is commonly used for this purpose. 



Class VIII 

 Heterocontae (^) 



In the Heterocontae a number of genera of green Algae are included which were 

 formerly placed in the Chlorophyceae but are now separated as an independent class. 



At the base of the class may be placed 

 such true Flagellata as Chloraijwcha hricro- 

 rtiorpha (Fig. 270), which lives in fresh 

 water. The naked, amoeboid cell contains a 

 nucleus and 2-6 yellowish-green chloroplasts ; 

 at the anterior end, just outside a vacuole, 

 two cilia are situated which are of very un- 

 equal length. It is this last feature which 

 gives the name to the group. Chlorainocha 

 is one of those low green organisms which 

 become colourless and lose the power of 

 independent nutrition when cultivated in a 

 nutritive solution in the dark. Thick-walled 

 resting cells also occur. 



Among the higher Heterocontae char- 

 acterised by the presence of pectin in the 

 cell wall the genus Conferva which is widely spread in fresh water must lie 

 mentioned (Fig. 271). The plant consists of simple unbranched filaments the 

 cells of which have peculiarly constructed walls ; the wall consists of two parts 

 separated by an oblique annular sjilit at the middle ])art of the cell. On cell 

 division a new portion, H-shaped in longitudinal section, is intercalated. The 

 characteristics of tiie group and the resemblance to Chloramoeha are seen in the 



Fii:. 270. — Chloramocba heteromorpha : v 

 vacuole ; I; uueleus ; 3, a rusting cell 

 (Aftrr Boiii.ix, from Oltmanns' Alr/ae.) 



