ALiG/E, QiQi'i 



coloured substance, Diatomine or Phycoxanthine. One of the most prominent pecu- 

 liarities of the Diatoms consists in their silicified cell -wall being composed of two 

 separated halves or valves of unequal age, of which the older one is pushed on to the 

 younger like the lid of a box. When the cell begins to divide, the valves separate 

 from one another, and after the division of the contents into two daughter-cells, each 

 of them forms a new layer on their plane of division which is adjusted by its turned-in 

 margin (the girdle) to the girdle of the old valve of the mother-cell ; this latter extends, 

 like the lid of a box, over the newly formed valve; and the two valves of the two 

 daughter-cells lie next one another. Since, according to Pfitzer, the silicious valves, 

 which also contain some organic matter, do not grow, it is clear that the new cells 

 must always become smaller from generation to generation. When they have thus 

 attained a certain minimum size, large cells, the Auxospores, are suddenly formed ; the 

 contents of the small cells, leaving the silicious valves which fall away from one another, 

 increase either simply by growth or by both conjugation and growth. After this 

 the auxospores surround themselves with new valves. Since the large auxospores are 

 of somewhat different shape to their smaller mother-cells and primary mother-cells, 

 the first result of their division must necessarily be cells of a different form and with 

 unequal halves, as in the Desmidieae (Fig. 157). The origin of the auxospores has been 

 more exactly followed out only in a few cases. It would appear that they are formed 

 in very different ways, from two or from one mother-cell, simply or in pairs, with or 

 without conjugation ; they are alike only in so far that their size greatly exceeds that 

 of the mother-cell. Diatoms are found in enormous numbers at the bottom both of 

 the sea and of fresh water, and attached to the submerged parts of other plants. Besides 

 the ordinary rotation of protoplasm in their interior, they also exhibit a creeping motion 

 by means of which they crawl over hard bodies or push small granules along their surface. 

 This occurs only in a line drawn along the length of the cell- wall, in which Schultze^ 

 supposes crevices or holes through which the protoplasm protrudes ; and this, although 

 not yet actually seen, probably occasions the creeping motion. 



The genus Vaucheria- must be considered somewhat more in detail, as the best- 

 known representative of a larger group, the Siphoneae, which, in the mode of growth of 

 their thallus, are nearly related to the Conjugatae, but whose mode of reproduction is 

 not yet sufficiently known. The thallus of Vaucheria consists of a single sac-like cell, 

 variously branched, often several inches or a foot long, containing no nucleus, and 

 developing on damp shady earth or in water. The fixed end is hyaline and branched in 

 a wavy manner ; the free part contains within the thin cell-wall a layer of protoplasm 

 rich in chlorophyll-grains and drops of oil, and enclosing the large sap-cavity. This part 

 of the thallus forms one or more main branches or stems which again branch beneath 

 their growing point (j) ; only in F, tuberosa the branching is also dichotomous ; though 

 commencing monopodially the lateral branches often develope sympodially. Besides the 

 occasional multiplication by the separation of branches or the regeneration of separated 

 pieces of the thallus, reproduction is also brought about by asexual spores and by sexually 

 produced oospores, and in such a manner that a long series of asexual generation usually 

 proceeds from the latter, until at last sexual plants arise from asexual spores ; but the sexual 

 generation can also produce spores as well as oospores. The spores may be produced 

 in very different ways, from the simple detaching of the end of a branch to the formation 

 of zoospores. In T. tuberosa lateral branches (sometimes also forked branches) swell up 

 to a considerable size through becoming filled with cell-contents, separate at the base, and 

 put out one or more germinating tubes. In T. geminata the end of a branch swells up to 

 an oval shape ; its accumulated contents become separated by a septum ; it contracts, and 



* [See Pop. Sci. Rev. 1866, p. 395.] 



" Pringsheim, Ueber Befruchtung und Keimung der Algen. Berlin 1855, and Jahrbuch fiir 

 wissen. Bot. II. p. 470.— Schenk, Wiirzburger Verhandl. vol. VIII. p. 235.— Walz, Jahrbuch fur 

 wissen. Bot. V. p. 127. — Woronin, Bot. Zeitg. 1869, nos. 9, 10. 



