Flowers. 191 



a stimulus is transmitted to the column resulting in the 

 sudden rupture of the tissues connecting the viscid disk 

 with the rest of the column. The viscid disk is then jerked 

 forward by means of the elastic band with sufficient force 

 to pull the pollen masses, termed pollinia, from their pollen 

 sacs and hurl them forward, disk foremost, to a distance of 

 one or even three feet ; but if the insect is standing in the 

 line of projection, which would probably be the case, the 

 viscid disk is thrown against its head or thorax and sticks 

 there. Thus the insect is made to carry away the pollen 

 masses, which are sticking out forward as the insect enters 

 the next flower. 



The flower just described is known as the male flower, 

 that is the pistil is abortive and doss not bear seed. If 

 the insect next visits a female flower, having a perfect 

 pistil but abortive stamens, the polHnia will be thrust into 

 a concave structure known as the stigmatic chamber, which 

 has an adhesive surface capable of holding the pollinia 

 with sufficient tenacity to wrest them from the insect. 

 Then the pollen tubes grow into the ovary and cross fertiU- 

 zation of the eggs is accomplished. 



In this flower we see a marvelous correlation of modi- 

 fied parts to attain a definite purpose ; but most wonderful 

 of all is the sudden transmission of a stimulus due to the 

 touch of the insect at one definite part of the flower, and a 

 correspondingly sudden dissolution of the tissues which 

 hold the viscid disk in place. 



140. Cross Pollination of Asclepias. — In the genus Ascle- 

 pias, we find an adaptation to cross polUnation by insects 

 scarcely less wonderful than that of the orchids. Asclepias 

 cornuti (Fig. 107), common everywhere in this country, is 

 perhaps the. best species for demonstrating this. As shown 

 in Fig. 108 the sepals {s) and petals (/) are reflexed ; the 



