FLORAL FOEMS AND THEIE RELATIONS TO INSECTS 349 



which blooms in moist places from July to September. Perhaps it is 

 not the best instance that might be given of the globular form some 

 of the Heaths (Erica) would better fulfil the requirements of the name 

 but we have adopted it because the flower is one of those which, 

 unlike the flowers which we have been hitherto considering, lay them- 

 selves out for pollination by insects with short probosces. Wasps are 

 its chief visitors, and easily reach down to the honey at the base of 

 the corolla : while long-tongued insects, such as humble-bees and butterflies, 

 avoid the flower altogether. 



The globular form 

 often merges impercep- 

 tibly into the urceolate, 

 or urn-shaped indeed, 

 a single spray of Heath 

 will sometimes show 

 both forms (fig. 431) 

 but a better example 

 of an urceolate flower 

 than the Heath is the 

 Whortleberry (Vacci- 

 nium myrtillus), the 

 corolla of which might 

 have furnished designs 

 to the sepulchral-urn- 

 makers of ancient 

 Etruria (fig. 422). As 

 a rule, only insects with 

 a proboscis long enough 

 to reach from the ex- 

 terior to the base of the 

 corolla, where the honey 

 is lodged, can reach 

 that coveted treasure, 

 as the opening is too 

 narrow to admit the 

 bodies or even the heads 

 of the majority of 

 nectar-sipping insects. 



A curious and exceptional form of corolla is the mitraeform or mitre- 

 shaped (fig. 423). It occurs in the Grape Vine (Vitis vinifera). The five 

 petals, which are coherent at their tips, form a dome-like covering to the 

 stamens and ovary ; but they are green and insignificant, and hardly dis- 

 . tinguishable from the foliage ; on which account they are valueless as insect 

 lures. The plant, therefore, does well in getting rid of them as soon as possible, 



Photo by] [E- Step. 



FIG. 429. YELLOW TOADFLAX (Linaria vulgaris), 



With personate or mask-like corolla. The upper and lower lips close 

 tightly to keep out alUinsects but bees. 



