INSECTS AND FLOWERS 453 



them to the flower and when there to the nectar and pollen 

 in such a way as to insure their brushing against either 

 the stamens or stigmas; third, modifications of shape so as 

 to prevent the stealing of nectar and pollen by non-helpful 

 insects; and fourth, blossoming at those times in the year 

 when the particularly helpful insects are most numerous, 

 and the opening of flowers at such times, in daylight, twi- 

 light or at night, as exactly to accord with the food seeking 

 times of the insects. The great variety in the shape, color 

 and patterns of flowers suggests the great variety that actually 

 exists in the ways in which flowers attract and at the same 

 time profit by the insects. 



The nectar, which is what most insects come to the flowers 

 to get, is a sweetish liquid secreted by special tissues called 

 nectaries. These may occur on any part of the flower, 

 but they are most frequently found at the bases of the stamens, 

 petals, and ovaries, and rarely on the calyx. In the plum 

 and peach they form a thick inner lining of the cup-shaped 

 receptacle. In nasturtiums the nectar is secreted in a 

 long spur from the calyx. 



Some flowers of simple construction expose their nectar 

 freely to all sorts of insects, but others conceal it in various 

 ways so that it is accessible only to insects of certain kinds. 

 A frequent device is to have some parts of the corolla close 

 the way to the nectar so that small insects which would not 

 assist in cross-pollination are excluded, and only those 

 which are strong enough to push aside the barrier or have 

 proboscides of proper construction to thrust past it can 

 obtain nectar and accomplish the transference of the pollen. 



The pollen, which is sought for only by bees and a few 

 little beetles, is a normal product of the flower and it is 

 only necessary that there be enough of it to supply the 

 insects and yet supply the plant's own use for fertilization. 

 The oldest and most primitive means developed among 

 plants to effect cross-pollination, a means still used by all 



