284 REPRODUCTION 



ing another flower, the back of the bee with the pollen upon it 

 comes first into contact with the lower half of the bifid stigma 

 (s) which projects a short distance below the anthers, and cross- 

 pollination is readily effected. Pollen from this second flower 

 is removed on leaving and transferred to a third, and so on. The 

 tongues of flies and other insects whose bodies are not stout 

 enough to fill up the mouth of the corolla, and come in contact 

 with the anthers, are too short to reach the honey ; moreover, a 

 ring of hairs (r) arranged across the lower part of the corolla 

 tube prevents small insects from robbing the flower of its nectar. 

 Almost all zygomorphic flowers, such as the bean, clover, sainfoin, 

 mint, snapdragon, and many others, exhibit striking adaptations 

 to secure cross-pollination by the agency of insects, and many 

 of these, when insects are prevented from visiting them, are 

 practically unable to effect self-fertilisation, and hence produce 

 little or no seed under such circumstances. 



It must, however, be mentioned that although many flowers, 

 such as those of the broad-bean, broom, carnation, red clover 

 and foxglove, are either unable to produce seed, or produce btit 

 few, when insects are excluded, others which show special adapta- 

 tion for cross-pollination by insects, and which are usually and 

 most advantageously pollinated by these agents, have also the 

 power of self-fertilisation, and often exercise it in dull weather, 

 or at other times when insect-visitors are scarce. For example, 

 the flowers of vetch, pea, dwarf-bean {Phaseolus vulgaris) and 

 tobacco produce seeds when specially protected from being 

 cross-pollinated by insects. Many protogynous flowers in a 

 young state are adapted for cross-pollination, but if the latter 

 does not take place, the stigma frequently receives pollen from 

 its adjoining anthers at a later stage of development of the 

 flower. 



Ex. 170. — Examine the following wind-pollinated flowers : —grasses, sedges, 

 rushes, oak, walnut, birch, alder, hazel, hop, plantain and dock 

 Note (i) the absence of conspicuous calyx or corolla. 



