146 PLANT BIOLOGY 



fall of its own weight on the adjacent stigma, or it 

 may be carried from flower to flower by wind, insects, or 

 other agents. There may be self-pollination or cross-pol- 

 lination, and of course it must always precede fertilization. 

 m. Usually the pollen is discharged by the burst- 



( \ ing of the anthers. The commonest method of 

 discharge is through a slit on either side of the 

 anther (Fig. 193). Sometimes it discharges 



through a pore at the apex, as in azalea (Fig. 

 Fig. 196.— & * I & 



anther of 196), rhododendron, huckleberry, wintergreen. 



Azalea, j n some plants a part of the anther wall raises 



opening by 



terminal or falls as a lid, as in barberry (Fig. 197), blue 



pores. cohosh, May apple. The opening of an anther 



(as also of a seed-pod) is known as dehiscence (dc, from ; 



hisco, to gape). When an anther or seed pod opens, it is 



said to dehisce. 



Most flowers are so constructed as to increase the chances 

 of cross-pollination. We have seen that the stigma may 

 have the power of choosing foreign pollen. The 

 commonest means of necessitating cross-pollina- 

 tion is the different times of maturing of stamens 

 and pistils in the same flower. In most cases 

 the stamens mature first : the flower is then 

 proterandrous. When the pistils mature first, 

 the flower is proterogynous. (A jut, and/; is a 

 Greek root often used, in combinations, for sta- barberry 

 men, and gyne for pistil.) The difference in stamen, 



r . . . with anther 



time of ripening may be an hour or two, or it opening by 

 may be a day. The ripening of the stamens lids - 



and pistils at different times is known as dichogamy, and 

 flowers of such character are said to be dichogamous. 

 There is little chance for dichogamous flowers to pollinate 

 themselves. Many flowers are imperfectly dichogamous — 



