THE STAMEN AND POLLEN-SAC 



247 



drying as the anther ripens is that their thin outer waUs shrinlc, while 

 the thick inner wall retains its form. The result of the shrinkage 

 of the outer side of the fibrous layer will naturally be to reduce the 

 curvature of the convex sporangia! wall, and the slit gapes widely 

 (tig. 192, B). The dusty pollen-grains are then readily removed. 

 There are various differfences in detail of the dehiscence of anthers, 

 particularly in highly specialised flowers. But in all ordinary flowers 

 where the pollen is dry and powdery, the way in which it is set free is 

 essentially like that described. 



The pollen itself varies greatly in dift'erent plants, in the form and 

 size of the grain, in the sculpturing of its walls, in colour, and in the 



I'lG. I'M. 



PoUeu-tetrads and poUen-graiiis of Callha. (i.) a teirad, with each cdl uiiiiiuclfatc. 

 (ii.) an older tetrad, with the grains almost separated, each containing two nuclei, 

 (iii., iv.) mature pollen-grains, showing the larger vegetative, and the smaller 

 antheridial mother-ceU. { -: ^^o.) F. O. B. 



dryness or stickiness of its surface. Bat these differences are only 

 external. There is a remarkable uniformity of the internal structure of 

 the pollen-grains of Flowering Pla)its (Fig. 194, iii. iv.). They are tico- 

 celled. One cell is usually the larger, and it is called the vegetative cell, 

 because it is from it that the pollen-tube is formed, and it does not take 

 any direct part in the reproductive process. It consists of cytoplasm, 

 and a nucleus. The smaller cell is the antheridial mother-cell ; it also 

 has cytoplasm and a nucleus, and is marked off from the other by a 

 plasmic film, not by a cell-wall. The two-celled state of the pollen- 

 grain may be attained while it is still in the unopened pollen-sac. 



The early stages of segmentation of the stamen may be followed by study 

 of the flowers of Chrysanthemum (Fig. 195, i.-iv.) . The stamen first appears as a 

 rounded papilla of tissue. It is covered b)' a laver of cells which divide onl)- 



