170 



MANUAL OF BOTANY 



Fig. 924. 



1 



stitiiting the external wall of the pollen sac. Both these layers 

 become composed of many cells, owmg to radial or anticlinal 

 divisions of those first formed. The innermost cell divides into 

 a variable number of cells, which are the mother cells of the 

 microspores or pollen grains. Each mother cell gives rise to four 

 special mother cells ; in the Monocotyledons by ordinary cell- 

 division twice repeated, the plane of the second division being at 

 right angles to that of the first : in the Dicotyledons the four 

 special mother cells are arranged at the four angles of a tetra- 

 hedron, as described in the case of the ferns. Each special 

 mother cell gives rise to a pollen grain, or microspore, by a 



process of rejuvenes- 

 cence. The tapetum, 

 and frequently the 

 walls of the original 

 mother cells, become 

 disorganised, forming 

 a sort of mucilagi- 

 nous fluid in which 

 the spores float. As 

 they mature their 

 walls are thickened 

 from within and usu- 

 ally form two coats, 

 the intine and the 

 exine. The exine 

 is often curiously 

 marked with spines 

 or ridges, due to 

 the deposition of 

 matter upon it from the disorganised tapetal cells. Eventually 

 the spores lie free in the cavity of the sporangium. 



Dehiscence is secured by the hygroscopic character of the 

 wall of spirally thickened cells described above, which under 

 different atmospheric conditions ruptures either by longitudinal 

 or transverse slits, or occasionally by a pore at or.near the apex. 

 The microspore so formed is a rounded, or oval, or rarely an 

 elongated body, containing protoplasm and nucleus, and a quantity 

 of reserve food material consisting of proteids, starch, oil, &c. It 

 has usually two coats, of which the outer is hard and thickened, 

 the inner thin and delicate. In some of the Gymnosperms the 

 outer coat is expanded at two places at the base of the spore to 

 form two bladder-like bodies which are filled with air. 



Fig. 924. Development of pollen in the 

 stamen of Lavatera. 1, 2. Yonug stages. 

 1. Transvn-se, 2. Longitmlinal, section of 

 anther, a, d. Tapetnm. h, c. Sporogeiious 

 cells. 3. Later stage. /. Tapetum. e. Four 

 special mother cells of the pollen. After 

 Dodel-Port. 



