D1V. I 



THALLOPHYTA 



391 



provided with two long cilia with knob -like thickenings at the tip (Fig. 322 

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

 preserved material of Corethron valdiviae that the inicrospores conjugated in pairs 

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

 daughter cell had at first 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 Closterium among the Desmidiaceae. 

 The ciliated gametes or microspores which have been observed in other genera 

 indicate a phylogenetic relation of the 

 Diatomeae to the Flagellatae, especi- 

 ally to the Chrysomonadinae. y ^LA f] 



Order 2. Diatomeae Pennatae 



In shape these are elongated, ellip- 

 tical, or boat-shaped^but may be wedge- 

 shaped ; the valves have their sculptur- 

 ing pinnate (Figs. 319, 323, 324).- In 

 many of the Pennatae (Fig. 79) a longi- 

 tudinal line corresponding to an opening 

 in the cell walls, and exhibiting swollen 

 nodules at both extremities and in the 

 middle, is distinguishable in the surface 

 of the valves. Forms provided with such 

 a median suture or RAPHE are character- 

 ised by peculiar creeping movements, re- 

 sulting from the streaming protoplasm 

 in the longitudinal slit of the raphe. 



The formation of the auxospores is 

 accomplished in various ways. The 

 process in Navicula, Pleurosigma, etc., 

 may be taken as a starting point ; two 

 cells lay themselves side by side, and 

 secrete a mucilaginous sheath. The 

 nuclei of these cells undergo a reduction 

 division, into four nuclei, two of which 

 are large and two small. Each cell 

 then divides into two gametes, each 

 containing a large and small nucleus. 

 The gametes escaping from the cell 



walls conjugate in pairs to form zygotes with four nuclei ; the two large nuclei 

 fuse, while the small nuclei disappear. Each zygote grows within a thin invest- 

 ment (perizonium) to form an auxospore several times the original size. This 

 secretes two new valves and commences to divide vegetatively (Fig. 323). 



In Surirella and Cocconels (Fig. 324) the conjugating cells do not undergo 

 division, but unite directly with one another. The nuclei, however, divide ; in 

 the former genus two nuclear divisions occur in each of the conjugating cells, 

 resulting in one large and three small nuclei ; in the latter genus there is only 

 a single nuclear division giving one large and one .small nucleus. The large nuclei 

 fuse, the small ones degenerate. 



In Achnanthes subsessilis the cell contents of a single cell divides into two 

 daughter cells, which escape, and then fuse together to form the auxospore. 



2 C2 



E 



D 



pr. 323. Formation of auxospores in Navicula 

 i-iridv.la. A, Cell seen from the valve side. B, 

 Two cells lying alongside one another ; their 

 contents have divided into two daughter cells, 

 each of which possesses two nuclei. C, D, 

 Conjugation in pairs of the daughter cells to 

 form the auxospores, which at first contain 

 four nuclei. . E, The two full-grown auxospores. 

 (x 500. After KARSTEX.) 



