CRYPTOGAMS 



351 



cell Division into numerous small, round, naked daughter cells. According to 

 KARSTEN'S observations in Corethrbn, these cells escape, conjugate in pairs, the 

 resulting zygotes increase in size, and ou germination give rise to two daughter 

 cells. Each of the latter has at first two nuclei, one of which later disappears, 

 and the cell grows into a mature individual of Corethron. These processes call to 

 mind the behaviour of Closterium among the Desmidiaceae ( 18 ). 



Countless numbers of Diatoms live in the ocean, and they constitute also a 

 proportionately large part of the PLANKTON ( 12 ), that is, the free-swimming organic 

 world on the surface of the sea. The plankton Diatoms are mostly centric forms, 

 have no middle suture or raphe on the surface of their valves, and are especially 

 adapted to swimming or floating. To this end they are often provided with horn- 

 like protuberances or membranous wings, which greatly enhance their buoyancy 

 (Figs. 277, 278). 



Many Diatoms 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 been shown that some 

 colourless species of Nitzschia which occur in the sea are exclusively dependent 

 on organic substances for food, the reduction of their chromatophores being 

 complete ( 19 j. 



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. 



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 Chloramoeba hetero- 

 morpha (Fig. 279), 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. Chloramoeba 

 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 be 

 mentioned (Fig. 280). The plant consists of simple unbranched filaments the 



Fio. 279. Chloramoeba heteromorpha : ?, 

 vacuole ; k, nucleus ; 3, a resting <''!!- 

 (After BOHLIN, from OLTMANNS' Algae.) 



