4iS 



GENERAL PRINCIPLES 



851. The flower of the common milkweed (Asclepias) is 

 greatly modilied. The five stamens are all united around the 

 ovary and they alternate with five funnel-shaped nectaries. 

 Externally nothing can be seen of either anthers or stigmas, 

 hut alternating with the five nectaries are five narrow slits 

 which open slightly at the upper end of the pendant flowers. 

 These slits open into the stigmatic cavities. The pollen is not 

 powdery but adheres in masses similar to the pollinia of Galeor- 



FiG. 255. — Structure of the milkweed flower. (Figures A and B should be 

 inverted.) A, The whole flower; B, the upper part of A enlarged; C, corolla; 

 /, sHt opening into the stigmatic cavity; h, nectary. C, a pair of pollen masses 

 connected by a V-shaped appendage and the sticky disc {d). (From Folsom.) 



chis. There are five pairs of such pollen masses and the pairs are 

 united in the form of a letter V, with a sticky substance at the 

 point of the V. The apex of the V coincides with the lower end 

 of the slit and the pollen masses are embedded in pockets which 

 extend upward on either side of the stigmatic cavity. When the 

 bee is clinging ''head down" to the pendant flower his feet 

 readily sHp into the slits and are thereby guided to the sticky 

 apex of the V pollen mass. With a strong pull the foot is re- 

 leased, with the pollen masses attached. When later the same 



