314 



BOTANY 



and by their further development they re-establish the original size 

 of the cells. 



The formation of. auxospores is accomplished in various ways. In the case of 

 Melosira, a free-swimming genus whose cells are joined together in chains, the 

 single cells simply swell greatly in size, and secrete two new valves (Fig. 233, D). 



An altogether different mode of spore- 

 formation is exhibited by the isolated, 

 unattached cells of Cocconema lancao- 

 latum (Fig. 233, B). In this instance, 

 two cells place themselves together side 

 by side, and throwing off their valves, 

 surround themselves with an envelop- 

 ing gelatinous mass. Each naked 

 protoplast, without, however, under- 

 going conjugation, is then transformed 

 into a single large auxospore, which 

 ultimately becomes invested with a 

 new cell wall. In other genera true 

 conjugation occurs ; thus, in the case 

 of Himantidium pectinate (Fig. 233, 

 A), each auxospore is the result of the 

 conjugation of two individuals. On 

 Fro. 233.— Formation of auxospores. A, Himtmti- the other hand, in the formation of 

 ilium pectinate ( x 200) ; B, Cocconema lanccolatum the auxospores of Bpithemia turgida 

 f,xm);C,Epithemialurgida(x200);I>,2Idosim ,-p- 233 ^ h f the 00n i U g at ing 



(A-C after Smith; D after ). * , '„''-,■ -j ■ . X b u 6 

 frustules first divides into daughter 



cells, which then, fusing two and two 

 with the corresponding daughter cells of the other frustule, give rise to two auxo- 

 spores. The auxospores do not pass through a period of rest, but begin at once to 

 multiply by division. 



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

 portionately large part of the plankton, that is, the free-swimming organic world 

 on the surface of the sea. The plankton Diatoms 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, like the contrivances of seeds for a similar purpose, greatly enhance 

 their buoyancy. 



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

 deposits of Siliceous Eakth, 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 cus- 

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

 microscopes. Pleurosigma angulatum is commonly used for this purpose, and, with 

 a sufficiently strong lens, it is possible to distinguish on the surface of the S-shaped 

 valves a system of fine markings, forming a network of six-sided meshes to the 

 right and left of the raphe. 



varicms (X 250). 

 Pfitzer.) 



