126 



PLANT STUDIES 



out of the pollen-sacs or to be removed from them. The 

 devices used by flowers containing both stamens and carpels 

 to prevent self-pollination are very numerous, but most 

 of them may be included under the three following heads : 

 (1) Position. — In these cases the 

 pollen and stigma are ready at the same 

 time, but their position in reference to 

 each other, or in reference to some con- 

 formation of the flower, makes it un- 

 likely that the pollen will fall upon the 

 stigma. The stigma may be placed 

 above or beyond the pollen sacs, or the 

 two may be separated by some mechan- 

 ical obstruction, resulting in much of 

 the irregularity of flowers. 



In the flowers of the rose acacia and 

 its relatives, the several stamens and 

 the single carpel are in a cluster, en- 

 closed in the keel of the flower. The 

 stigma is at the summit of the style, 

 and projects somewhat beyond the 

 pollen-sacs shedding pollen. Also there 

 is often a rosette of hairs, or bristles, 

 just beneath the stigma, which acts as 

 a barrier to the pollen (see Fig. 131). 



In the iris, or common flag, each 

 stamen is in a sort of pocket between 

 the petal and the petal-like style, while 

 the stigmatic surface is on the top of a 

 flap, or shelf, which the style sends out 

 as a roof to the pocket. With such an 

 arrangement, it would seem impossible 

 for the pollen to reach the stigma un- 

 aided (see Fig. 132). 



In the orchids, remarkable for their 

 strange and beautiful flowers, there are 



Fig. 133. A portion of 

 the flower of an iris, 

 or flag. The single 

 stamen shown is 

 standing between the 

 petal to the right and 

 the petal-like style to 

 the left. Near the 

 top of this style the 

 stigmatic shelf is 

 seen extending to the 

 right, which must 

 receive the pollen 

 upon its upper sur- 

 face. The nectar 

 pit is at the junc- 

 tion of the petal and 

 stamen. While ob- 

 taining the nectar the 

 insect brushes the 

 pollen-bearing part 

 of the stamen, and 

 pollen is lodged upon 

 its body. In visiting 

 the nest flower and 

 entering the stamen 

 chamber the stig- 

 matic shelf is apt to 

 be brushed. — After 

 Gray. 



