734 



PHYSIOLOGY. 



VP 



05^ § 



.0 





kjT 



Fiff. 1102. Yeast plant in process of 



development. Fig. 1103. Conferva 



glomerata, showing the progressive 

 stages of gemmation (b, c, d, e). 

 a. Terminal cell. After Mohl. 



In some of the lower kinds of plants, a modification of this 

 process of cell-division takes place ; it consists in the formation 

 of secondary cells, as little bud-like prominences on the primary 



cells, either at their ex- 

 Ficf. 1102. Fig. 1103. tremities, as in the Yeast 



plant (/^. 1102), &c., by 

 which the plant is in- 

 creased in length; or on 

 the side of the primary 

 cell when branches are 

 produced, as in some Con- 

 fervas (Jig. 1103), in the 

 fibrilliform cells of Fungi 

 and Lichens, and in other 

 cases ; probably much 

 more frequently than is 

 commonly supposed. The 

 mode in which this bud- 

 ding occurs may be thus 

 described. At a certain 

 point the primordial utri- 

 cle appears to acquire a 

 special development, for 

 it is seen to bulge out, 

 carrying the cellulose wall 

 of the cell before it, by which a little prominence is produced 

 externally {Jig. 1103, b); this continues to elongate until it 

 forms a tubular projection, c, on the side of the primary cell. 

 The cavity of this projection is at first continuous with that 

 of tlie cell from whence it sprung, but after it has acquired a 

 certain definite length, its primordial utricle becomes constricted 

 at the point of contact with the primary cell, d, and ultimately 

 forms a partition between them, as in the ordinary process of 

 cell-division. This process of cell-division is termed gemmation 

 or budding. In some cases, as in the formation of the fibrilliform 

 cells of Fungi and Lichens, no partitions are formed, but all the 

 branches communicate with each other {Jig. 26). 



b.- Cell-division with absorption of the walls of the parent cell, 

 and the setting free of the new cells. — The pollen cells in the 

 anthers of all Flowering Plants, and the spores of most Flower- 

 less Plants, are formed bj' this process, which only occurs in 

 connection with the organs of reproduction. The manner in 

 which it commonly takes place in the formation of pollen is as 

 follows : — the primordial utricle of each parent cell becomes 

 infolded so as to divide the protoplasm into four portions, 

 either directly, or indirectly by first dividing it into two, and 

 then each of these being again divided into two others ; these 

 four portions are called special parent cells ; the whole of the 



