Arrangements for Securing Union of Sexes 309 



trees, especially the cone-bearing kinds, are particularly abundant, 

 the Uttle lakes of the woods are covered in the spring time with 

 pollen enough to make a continuous film all over the surfaces, 

 while of course an equal amount must fall on the land. So plenty 

 at times is the pollen in the air of those countries that it receives 

 the expressive appellation of ''sulphur-shower." Now pollen, 

 composed as it is almost wholly of the richest protoplasmic ma- 

 terial, is one of the most difficult and expensive of substances 

 for plants to manufacture; and therefore the wastefulness of wind 

 pollination must entail a great drain on these plants. Obviously, 

 any method which would ensure the transfer of pollen direct 

 from the anthers of one plant to the stigmas of another would 

 be greatly superior in both economy and certainty to wind pol- 

 lination. Such a method, indeed, plants have developed; and it 

 consists in the utilization of the locomotive powers of animals, 

 especially insects. 



We turn, accordingly, to the study of the cross pollination of 

 flowers by insects. Obviously a first requisite of the method is an 

 arrangement that will lead insects to go directly from flower to 

 flower, — a thing which they will not do unless induced by some 

 attraction or compulsion. The inducement takes the form of a 

 store of nectar,— a sugary liquid both nutritious and palatable 

 to insects, and easily made by plants in little superficial glands. 

 These nectar glands, which often pour their product into special 

 receptacles called nectaries, and which exhibit a great variety of 

 forms in different flowers, are of course placed in close juxtaposi- 

 tion to the stamens and pistils (figure 110). They constitute the 

 most fundamental feature of insect-pollinated flowers; and those 

 plants which possess them along with stamens or pistils but no 

 other parts, for example the Willows and some Maples, represent 

 a first stage in the evolution of the insect-pollinated flower. But 

 a second requisite of the method is some arrangement by which 

 the position of the inconspicuous nectar (and therefore of the 

 stamens and pistils), can be made evident to the insects; and this 



