128 



PLANT RELATIONS. 



stigma of the same flower are not mature at the same time. 

 It is evideut that this is a very effective method of prevent- 

 ing self-pollination. When the pollen is being shed the 

 stigma is not ready to receive, or when the stigma is ready 

 to receive the pollen is not ready to be shed. In some 

 cases the pollen is ready first, in other cases the stigma, 

 the former condition being called protandry, the latter 

 protogyny. This is a very common method of preventing 



self-pollination, and is com- 

 monly not associated with 

 irregularity. 



The ordinary tigwort may 

 be taken as an example of 

 protogyny. When the flowers 

 first open, the style, bearing 

 the stigma at its tip, is found 

 protruding from the urn-like 

 flower, while the four 

 stamens are curved down 

 into the tube, and not ready 

 to shed their pollen. At 

 some later time the style 

 bearing the stigma wilts, 

 and the stamens straighten 

 up and protrude from the tube. In this way, first the 

 receptive stigma, and afterwards the shedding pollen-sacs, 

 occupy the same position. 



Protandry is even more common, and many illustrations 

 can be obtained. For example, the showy flowers of the 

 common fireweed, or great willow herb, when first opened 

 display their eight shedding stamens prominently, the style 

 being sharply curved downward and backward, carrying 

 the four stigma lobes well out of the way. Later, the 

 stamens bend away, and the style straightens up and ex- 

 poses its stigma lobes, now receptive (see Fig. 134). 



(3) Difference in pollen. — In these cases there are at 



Fig. 134. Flowers of fireweed (Epi- 

 lobivm), showing protandry. In 1 the 

 stamens are thrust forward, and the 

 style is sharply turned downward and 

 backward. In 2 the style is thrust 

 forward, with its stigmatic branches 

 spread. An insect in passing from 1 

 to 2 will almost certainly transfer pol- 

 len from the stamens of 1 to the stig- 

 mas of 2.— After Gray. 



