32 NATURE SKETCHES IN TEMPERATE AMERICA 



effect cross-fertilization. Darwin was th6 first to demonstrate 

 that the vigor and fertility of the next generation of plants 

 were greatly increased by this process. It was, moreover, 

 this discovery that led to researches which disclosed the most 

 wonderful and complex arrangements that exist in flowers, 

 all having for their object the prevention of constant self- 

 fertilization; but that pollen shall be carried, either constantly 

 or occasionally, from the flowers of one plant to those of another. 

 There was thus estabhshed the fact that the arrangement, 

 length, and position of all the parts of the flower have a definite 

 purpose, though a great variety of ways exist by which this 

 same result is obtained. Open cup-shaped and quite regular 

 flowers, in which it seems inevitable that the pollen must fall 

 on the stigma and produce constant self-fertihzation, are often 

 prevented from doing so by a physiological variation. In 

 these cases the anthers, continually emitting their pollen, wither 

 either a little earlier or a little later than the stigmas of the 

 same flower or of other flowers on the same plant when in the 

 best state to receive it. As individual plants differ somewhat 

 in the time of flowering, the pollen of one plant would often be 

 conveyed by insects to some other plant whose stigmas were in 

 a proper condition to be fertilized by it. 



Variations occurred in first one part, then in another, which 

 have resulted in various adaptations for insect fertilization. 

 Odors are developed as an attraction or guide to insect fertili- 

 zation. Inconspicuous flowers are often possessed of strong, 

 sweet odors, to be detected some distance away, while very 

 showy flowers are seldom thus provided with scents. White 

 flowers are usually exceedingly sweet perfumed. They are 

 mostly fertilized by night-flying moths. 



The grouping of flowers so that they attract insects is often 

 of considerable advantage. They are often conspicuously dis- 

 played in a broad flat top, such as is exhibited in the elderberry 

 and wild carrot. These groups are made up of many individual 

 flowers. Then there is the grouping shown in those of the 

 lilacs and horse-chestnut. Again, we witness them closely 

 packed together in tiny florets forming dense heads, as in the 

 clover and all the Compositae, and in these the marginal 

 florets are modified into rays such as shown in the daisy, aster. 



