830 



ECOLOGY 



1142 



1143 



1144 



Figs. 1142-1144. — Stamens of angiosperms, 

 showing methods of anther dehiscence: 1142, 

 ordinary stamens with longitudinal dehiscence; 

 1143, o. stamen of Solanum with dehiscence by a 

 terminal slit or pore; 1144, a stamen of Vaccinium 

 with tubular prolongations of the pollen sacs. — 

 From Kerner. 



in the Ericaceae, figs. 1143, 

 T144). Dehiscence is occa- 

 sioned by tissue desiccation. 

 Beneath the epidermis is a 

 layer with unequally thick- 

 ened fibers, in which strains 

 arise when the water content 

 lessens; rupture then occurs 

 along the Unes (or at the 

 spots) of weakness, where- 

 upon the pollen may be 

 shaken out by such agents 

 as wind and insects. 



Commonly anther desiccation 

 is due to the great transpiration 

 to which open flowers are exposed. 

 Some anthers, however, open in 

 the bud or in moist weather and it has been claimed that this is due to the absorption 

 of water from the anther by adjoining nectaries or by other tissues rich in sugar. 

 Dehiscence occurs when anthers are placed in contact with a cane sugar solution, 

 though much more slowly 

 than in dry air. Light and 

 the pressure of growing pol- 

 len also appear to facilitate 

 dehiscence. 



The pollen. — The 

 pollen grains are borne 

 in pollen sacs within the 

 anther, where they com- 

 monly are produced in 

 fours {tetrads). Usually 

 the grains break apart at 

 maturity, scattering in- 

 dependently, but in some 

 plants they cohere in 

 groups (as in Mimosa), 

 while in others they 

 cohere in large and defi- 

 nite masses, known as 

 pollinia (as in the milk- (exine); highly magnified. — From Merrell. 



1143 



1147 



Figs. 1145-1148. — Different stages of pollen grain 

 development in a rosin-weed (Silphium) : 1145, a micro- 

 spore, representing the one-celled stage of a developing 

 pollen grain; 1146, a mature pollen grain; 1147, 1148, 

 germinating pollen grains, shoving the first stages of pol- 

 len tube development ; note the thick and spiny outer coat 



