. A handbook of cryptogamic botany. Cryptogams. 422 PROTOPHYTA the genus Navicula, possess the power of propelling themselves through the water with considerable rapidity backwards and forwards in the direction of their longer axis, often with a jerking motion, or of creeping along the bottom on some submerged substance. The cause of this motion is a subject on which a large amount of attention has been be- stowed. Nageli attributed it to osmotic currents passing through the cell-wall. Ehrenberg believed that he had actually seen, in some cases, the extrusion through the raphe of vibratile cil
. A handbook of cryptogamic botany. Cryptogams. 422 PROTOPHYTA the genus Navicula, possess the power of propelling themselves through the water with considerable rapidity backwards and forwards in the direction of their longer axis, often with a jerking motion, or of creeping along the bottom on some submerged substance. The cause of this motion is a subject on which a large amount of attention has been be- stowed. Nageli attributed it to osmotic currents passing through the cell-wall. Ehrenberg believed that he had actually seen, in some cases, the extrusion through the raphe of vibratile cilia, in other cases of a ' foot' or pseudopode; but his observations have not been confirmed by others. The explanation of the mo- tion now generally accepted is that of Schultze—that it is due to the contractility of the protoplasm which is exuded outside the cell-wall. Mereschkowsky (Bot. Zeit., 1880, p. 529) states the arguments in support of the various views with regard to the causes of the mo- tion, and sums up in favour of the theory that it is the result of osmotic currents within the siliceous cell-wall. Hallier, again (Unters. iiber Diatomeen, 1880), considers it due to a contractile layer of protoplasm, and asserts that at an early stage di- atoms have no true cell- wall of cellulose. Onderdonk (Microscope, 1885, p. 205) also attributes it to 'external cyclosis.' Diatoms have three modes of multiplication :—by simple division, by auxospores, and by a kind of conjugation which is regarded by some as sexual ; but the three modes pass gradually one into another. Simple division always commences with the bipartition of the nucleus. When it is about to commence the two valves separate from one another, the contents divide into two daughter-cells, and new siliceous valves are formed inside the old ones, and therefore necessarily smaller than they. The valves of the new individual are formed necessarily one after the other, the one formed later being smaller. The individ
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