3°4 



FLOWERS 



Fig. 126. — Pollination by means 



of water as illustrated by the have different parents. 



plant. This is called geitonog- 

 amy. It is the commonest kind 

 of pollination accomplished by 

 insects. The principal pollinat- 

 ing insects usually explore 

 thoroughly all the flowers of one 

 plant before going to another. 



(2) The pollen may cross to the 

 stigmas of flowers of other plants 

 of the same kind. This is called 

 xenogamy. Evidently this is a 

 more complete kind of crossing 

 than geitonogamy is ; in fact, the 

 latter is more closely allied to 

 close-pollination than it is to 

 xenogamy. Close-pollination and 

 geitonogamy both mean that the 

 pollen parent and the ovule parent 

 of the seed are the same, while in 

 xenogamy the pollen and the ovules 

 It is evi- 



tape grass (Vallisneria). This 

 plant is dioecious. The individ- 

 ual at the left bears a spike 

 of staminate buds (A). These 

 spherical buds (a) become de- 

 tached and rise to the surface, 

 where, as they float, they open 

 and expose the stamens. The 

 pistillate flowers are borne on 

 long stems which come just to 

 the surface of the water. Polli- 

 nation is accomplished by those 

 staminate Jlowers which float 



against the pistillate ones. After pollination the stem of the pistillate flowers 

 coils into a spiral, withdrawing the ovary below the surface. The fruit develops 

 under the water. 



dent that all diclinous flowers 

 are cross-pollinated, also that all 

 diclinous flowers borne by dioe- 

 cious plants (see page 284) must 

 be xenogamic. That is, since 

 the staminate and pistillate 

 flowers of diclinous plants are 

 borne on different individuals, 



