58 PRACTICAL BOTANY. 



Diatoms are reproduced both sexually and asexually. 

 By the asexual process the cell divides lengthwise by the 

 separation of its two valves. Each of the valves develops 

 a new valve to mate itself, and after a time the two new 

 cells separate entirely. The new valve always develops 

 on the inside of its mate, and is therefore smaller. If this 

 diminution were to continue indefinitely, the Diatoms 

 would, of course, practically disappear in time. 



The sexual process of reproduction is evidently designed 

 to prevent this disaster. In this process the contents of 

 individual cells are set free from between the valves and 

 become zoospores. These zoospores unite in pairs, and 

 the resulting zygospore or auxospore develops into a large 

 form. (Fig. 28, 19, 20.) This new form is, however, 

 usually different from the parent form. By the sexual 

 process of reproduction nature not only prevents the ex- 

 tinction of the Diatoms, but provides for the production 

 of countless numbers of new species. A third process of 

 reproduction has been described by a few recent observers. 

 By this process the contents of cells are set free from be- 

 tween the valves, and each cell becomes an auxospore 

 without previous conjugation. 



Although distinct sexual and asexual forms occur, the 

 life history of these plants is not fully understood. The 

 alternation of generations is by no means clear. The ordi- 

 nary asexual forms may be potential gametophytes, or they 

 may be sporophytes. 1 



References for Reading. Bennett and Murray's " Cryptoganiic 

 Botany," pp. 419-426; Carpenter's "The Microscope and its Revela- 

 tions, "pp. 516-554; Goebel's "Classification of Plants," pp. 17-19; Stras- 

 burger and Hillhouse's "Practical Botany," pp. 210-213; Strasburger 

 Noll, Schenck, and Schimper's " Lehrbuch der Botanik," pp. 273, 274. 



1 The pupil should be given gatherings both of recent and of fossil 

 Diatoms for examination. He should also be encouraged to collect mate- 

 rial for himself. The teacher can easily arrange a series of Practical 

 Studies on any forms that are conveniently obtained in the locality. 



