UELL FORMATION BY DIVISION. 37 



then a partition is formed subdividing the sac ; the halves 

 either separate at once and each rounds itself ofE and becomes 

 an independent cell, or one or both halves again divide in a 

 similar way before they separate, and so three or four new 

 cells are produced." 



47. — In many of the filamentous Thallophytes a similar fis- 

 sion takes place, but in these the cells do not immediately sepa- 

 rate from one another after their formation. Thus, in Nostoc 

 and Oscillatoria (Fig. 26) the cells do not differ in any essen- 

 tial way as to their formation from those which constitute 

 Protococcus. In Nostoc after fission the cells round them- 

 selves up and retain but a slight and easily separable connec- 

 tion with one another ; in 

 Oscillatoria, on the con- ^^ ^ 



trary, the cells remain cy- 

 lindrical and are less read- 

 ily separable. 



48. — In Spirogyra (Pig. Fig. 26.—^, filament of mstoc ,- B, filament 

 „„ i-x n J! ot OscUlatoria. X 300.— After Prantl. 



36, p. 4o) new cells form 



by the partition of old ones. The protoplasmic sac infolds all 

 around the middle of the old cell which is cylindrical in 

 shape ; into the circular channel thus formed the cell-wall 

 extends, appearing at first as a narrow projection from the 

 original wall, but becoming broader and broader, until it 

 forms a complete partition. When the new cells have 

 elongated by intercalary growth the process of fission may be 

 repeated, and so on.* 



49. — The cells which make up the greater part of the 

 tissues of the higher plants are formed by fission. In the 

 apical cells of Equisetum we find a curious regularity in the 



* The student is referred to Saclis' " Text-Book," pp. 17-18, for a further 

 description of this process in Spirogyra; and to Von Mohl's " Anatomy 

 and Physiology of the Vegetable Cell," pp. 50-51, for a description of tlie 

 similar fission of Oladophora glomerata (Gonfena glomeratn, Linn.). Von 

 Mohl's description, which was the result of the first accurate investiira- 

 tionof cell-formation, is erroneous in this — that he supposes that durinji 

 the process, to quote his words, " a cellulose membrane is deposited all 

 over the outside of the primordial utricle" of the whole cell, and that 

 it is a portion of this new membrsine which forms the partition. 



