THE STAMEN AND POLLEN-SAC 



249 



The pollen-mother cells are all derived by division from the inner product 

 of the hypodermal cells. They vary in number in different anthers. Caltha 

 gives an average example, in which from 14 to 1 6 appear in each transverse 

 section of a pollen-sac (Fig. 196). They are characterised by very thin walls, 

 a dense, non-vacuolated cytoplasm, and a proportionately very large nucleus. 

 At first they are closely fitted together, without intercellular spaces (a). But 

 presently the pollen-sac distends, and the pollen-mother cells round themselves 

 off, and become suspended individually in a fluid medium which fills the spaces 

 between them (b). Meanwhile the tapetal cells, from which the fluid probably 

 arises, retain their form, though their nuclei often increase by fragmentation. 



a b 



FIG 196. 



a, 6, successive stages of development of the contents of the pollen-sac. See text, 

 (x 100.) F. O. B. 



As the pollen-mother-cells separate they enter on the tetrad-division. Each 

 nucleus divides first into two, and then into four. These rapidly repeated 

 divisions are characteristic of all spore-formations, and have an important 

 relation to the constitution of the nuclei themselves. The four resulting 

 nuclei are first enclosed in the single protoplast. But soon each is separated 

 by a partition-wall, and is surrounded by a quarter share of the cytoplasm. 

 The pollen- tetrad is thus constituted. The four cells are still enclosed by the 

 common wall ; but later each cell deposits a special wall round itself. The 

 common wall, which was of a mucilaginous character, is then dissolved, with 

 the result that the four cells become dissociated as independent pollen-grains 

 (Fig. 194, i. ii.). Meanwhile the single nucleus of each has divided, giving the 

 two nuclei present in the mature grain (Fig. 194, iii. iv.). 



This description of the development seen in Caltha applies for the develop- 

 ment of the pollen -in all ordinary Dicotyledons. In Monocotyledons the 



