25] 



CHAPTER I. THE CELL. 



85 



plants (Fig. 61), and in the formation of the spores of Mosses and 

 Vascular Cryptogams. The details of this process are not the same 

 in all cases. In some (development of the pollen-grains of Mono- 

 cotyledons and of the microspores of Isoetes) the nucleus of the 

 mother-cell divides into two, and this is followed- by a corresponding 

 division of the cytoplasm, a cellulose wall being formed between 

 the two cells. Each of these now divides in the same manner, in a 

 plane at right angles to that of the first division, and thus the four 

 special mother-cells are produced lying in one plane. In other 

 cases (development of the pollen-grains of Dicotyledons, of the spores 

 of Mosses, Ferns, and Ec[uisetums) the nucleus of the mother-cell 

 divides into two, and each of these secondary nuclei divides again 



FIG. 61. Division of the mother-cells of the pollen- 

 grains of Althaea ro*o. At A and B the division of the 

 protoplasm into four has begun ; in D the in-growth 

 of the membrane is shown, and in E the walls are 

 complete. (After Sachs.) 



FIG. 62. Rejuvenescence as ex- 

 hibited in the formation of the 

 zoospores of (Edogonium. A 

 Portion of a filament ; in the lower 

 cell the protoplasm is begin- 

 ning to contract, in the upper the 

 young primordial cell is escaping 

 (Z). B A zoospore. C The be- 

 ginning of germination. ( x 350.) 



into two, the divisions taking place in planes at right angles to each 

 other and to that of the first division ; as a consequence, the four 

 nuclei do not lie in one plane, but are arranged tetrahedrally. Cell- 

 walls are now formed, so that four special mother-cells are produced. 

 In the case of the pollen-grains of Dicotyledons, the wall of the 

 primary mother-cell thickens and grows inwards at certain points 

 (Fig. 61 Z>) so as to constrict the cytoplasm somewhat, and the 

 newly-formed walls become attached to these projections. In all 

 cases the protoplasm in each of the four special mother-cells 



