THE TOO-PERFECT MILKWEED 



Its Method of Covering Insect Visitors with Pollen, to Insure Proper Cross- 

 Fertilization of the Species, Appears to Have Been Carried beyond 

 the Height of Perfection, so That it Now Defeats the 

 Purpose for Which it is Intended 



IN HER attempts at specialization, 

 Nature is often startlinj;ly success- 

 ful, but there are a few instances, 

 among both animals and plants, 

 where this specialization has over- 

 reached the capacities of the organism 

 specialized, and thus the specialization 

 has defeated its own ends. The classic 

 example of this is, of course, the Irish 

 stag, whose highly developed antlers 

 enabled it to overcome its opponents in 

 the struggle for existence. If the devel- 

 opment of the horns had stopped at 

 this stage, all would have been well, 

 liut the horns kept on increasing in size 

 until they became so enormous that the 

 stag was unable to support them, and 

 the species finally came to extinction. 

 A more familiar, though less striking, 

 case of adaptation carried too far is 

 presented by the common milkweed or 

 silkweed of the eastern United States 

 (Asclcpias syriaca), whose specialized 

 flowers a])])ear to have develo])ed further 

 than is really desirable. Although this 

 over-specialization may be dooming 

 the 1)1 ant to eventual extinction, the 

 milkweed is at ])resent one of the 

 sturdiest and most ubiquitous weeds in 

 the eastern part of the United States, 

 and so far certainly gives no evidence 

 that it is losing out through any 

 hy]3ertro])hy. 



Due to the fact that it is never self- 

 fertilizing, the milkweed de])ends en- 

 tirely on insects for distributing the 

 pollen from plant to plant. Evidently, 

 if it was made too easy for insects to 

 secure nectar, they would drink their 

 fill and fly away without carrying much 

 pollen. It was an advantage to the 

 species to make the insect struggle in 

 the flower, thus thoroughly covering 

 itself with pollen which would lie 

 carried to the next blossom. 



To assure a struggle on the j^art of 

 the insect, the plant developed a sticky 



460 



juice which exudes through the petals. 

 This serves the double purpose of mak- 

 ing the insect 'move violently in order 

 to get away, thus bringing its feet and 

 legs in contact with a large quantity of 

 pollen, and also of coating the feet and 

 legs with the sticky substance, so that 

 the pollen grains would adhere more 

 readily. 



Thus far the plan would succeed 

 admirably, but the specialization was 

 not arrested at the height of its useful- 

 ness, but continued developing until 

 many insects are maimed, crippled or 

 killed by the action of the sticky 

 substance, as shown in Fig. 7. Thus 

 the plant appears to defeat its own ends 

 by causing the insects to be so crippled 

 that they are unable to carry on the 

 work of fertilization as was originally 

 intended, and furthermore, the large 

 deposit of pollen which is placed on the 

 insect's feet is lost, and hence not 

 a^■ailalJle to the next visitor. 



In the accompanying illustration (Fig. 

 7) is shown a flower with two legs, 

 Ijelonging to a recent visitor, still 

 sticking to the petals. It may easily 

 be seen that the former owner of these 

 legs would probably be of little further 

 benefit to milkweeds. Just above the 

 flower is pictured a fly with the third 

 leg on the left side gummed fast to the 

 left wing, so that it is impossible for it 

 to fl}', thus effectually preventing it 

 from fertilizing other flowers. F^'rom 

 these two instances, the disadvantages 

 accruing from this over-specialization 

 are self-evident. 



In the other picture (F^'ig. 8), two 

 sprays of the ]jlant are shown natural 

 size, so that it is more easily recognized 

 than in the photograph of a single 

 flower greatly enlarged. The spray on 

 the left shows the beautiful flowers, 

 while that on the left is a head of buds. 

 The stem is tall and stout, and fre- 



