126 



PRACTICAL BOTANY 



display is all made by an enlarged and conspicuous set of 

 specialized leaves (bracts) which surround the flower, as in 

 the flowering dogwood and many euphorbias (Fig. 292), or 

 even by highly colored ordinary leaves, as in the poinsettia. 

 120. Degrees of specialization for insect visitors. Flowers 

 with a spreading perianth and radial symmetry — like those 

 of the stonecrop (Fig. 92) and the live-forever, the buckwheat 

 (Fig. 114) and the caraway (Fig. 295), buttercups, poppies, 



roses, and hun- 



dreds of other 

 familiar kinds — 

 are open to all 

 comers, and are 

 frequented by 

 many sorts of in- 

 sects, from flies 

 to bees. 



Flowers with 

 bilateral symme- 

 try — like vio- 

 lets, wild balsam 

 (Fig. 119), most 

 flowers of the 

 Pea family (Fig. 98), mints, and many others — are usually 

 not suited to indiscriminate visitors, but only to those insects 

 which can get at the nectar, the pollen, or both. In violets, for 

 example, the pollen is abundant, but is concealed within the 

 throat of the corolla, and the nectar is deep down in the spur 

 of the corolla. Both pollen and nectar are easily reached b}- 

 the tongues of bees, but not by small insects. In the snap- 

 dragon the mouth of the corolla is firmly closed, so that small 

 insects cannot enter it. Larger ones, such as bees, can, how- 

 ever, readily overcome the elasticity of the hinge at the junc- 

 tion of the lips and enter the flower (Fig. 115). 



There are some flowers which appear to be dependent on 

 pollination by a single kind of insect only, and therefore are 



Fig. 115. Flowers of snapdragon 



A, with lips of corolla tightly closed ; JB, with the lips 

 forced open by a visiting bee 



