I3i6 A TEXTBOOK OF THEORETICAL BOTANY 



the other side of its body to the stigma the pollen which it had previously 

 collected will be wiped off on the stigmatic margin. The anthers them- 

 selves are peculiar. Initially they are extrorse, but open by two valves 

 situated at the sides. These valves with the pollen attached to them turn 



Fig. 1234. — Berberis vulgaris. A, Flowering shoot. B, Vertical section 

 of flower, early stage with stamens extended. C, The same, later 

 stage with anthers open and stamens contracted. D, Single sta- 

 men showing valvular dehiscence. E, Honey leaf of perianth 

 with two nectaries at its base. 



inwards so that finally the pollen faces towards the centre of the flower. 

 Should cross-pollination fail, self-pollination is possible by the anthers 

 coming into contact with the stigmatic surface, though there is considerable 

 doubt as to whether successful fertilization follows automatic self-pollina- 

 tion. 



3. Flowers with Concealed Nectar 



The flowers belonging to this class are the most highly specialized, 

 though they are connected with the last by many transitional forms. The 

 characteristic feature of the group is that the nectar is not merely out of 

 sight, but is structurally concealed either in pouches or sacs or by being 

 covered by hairs or other floral parts. Moreover these structures cover the 

 nectar even when the flower is fully expanded. Though many of the 

 flowers are actinomorphic there is a marked tendency towards zygomorphy 

 in this group. Similarly, though a few white and yellow flowers exhibit the 

 condition, by far the greater number of flowers with concealed nectar 

 are red, blue or violet in colour. 



Many of the flowers belonging to this class are visited only by a single 

 group of insects, some indeed only by a single species. Lythrum salicaria, for 

 example, is usually only visited by a single species of bee, which rarely 



