DISTRIBUTION AND ADJUSTMENT 



107 



in any one locality — that the group may be collectively called 

 the milkweed association. So we may speak of the com plant 

 association, a group of plants and animals that directly affect the 

 crop and so assume large practical significance. Such a grouping 

 of animals would, of course, 

 make as many societies as 

 there are food plants, a multi- 

 plication of detail that fortu- 

 nately is obviated by the fact 

 that plants are grouped into 

 societies by the interplay of 

 certain factors, so that the 

 animals, as far as the food 

 factor is concerned, tend to 

 a like grouping. This will 

 appear in detail in the suc- 

 ceeding chapters. 



This relation of a specific 

 animal to a particular plant 

 as its source of food is the 

 foundation of some of the 

 most remarkable adaptations 

 to be found in nature. The 

 nectar of the flowers, much 

 sought by insect epicures, is 

 available only to a favored 

 few. Thus the common weeds, 

 butter and eggs and the closed 

 gentian (Fig. 65), both bear blossoms that shut with so firm a 

 spring that only the strong and heavy bumblebee can force an 

 entrance. He pays for their hospitality by carrying the pollen 

 so essential to flower fertilization and seed production. A rosin 

 weed common on the prairie, Silphium perfolialum, bears water 

 cups, ensheathing the stem, formed by the bases of the leaves 

 (Fig. 66) . These effectually keep crawling insects away from the 



Fig. 65. — The closed gentian, Gcnliana 

 Andrewsii. 



