260 BOTANY. 



at its bottom; or they may be of different sizes, as in 

 orchids, beans, peas, etc., where they are so placed as to 

 admit of access to the nectar from one direction only. In 

 some tubular flowers there are two forms in the same spe- 

 cies, those of some plants having long stamens and short 

 styles, while in others the structure is exactly the reverse. 

 Insects in getting honey from these will pollinate the long- 

 styled flowers with pollen from the long stamens of other 

 flowers, and vice versa. There is also very often a greater 

 or less difference in the time of maturity of the stamens 

 and pistils. In some the pollen is set free before the 

 stigma is ready for pollination ; in others it is the reverse. 

 This (and the preceding) arrangement prevents pollination 

 of a pistil by pollen from the stamens of the same flower; 

 i.e., close fertilization is prevented. 



494. Self-pollinated (autogamous) flowers are much less 

 numerous than those which are wind- or insect-pollinated, 

 and it is doubtful whether there are any species of plants 

 all of whose flowers exhibit constant self-pollination (au- 

 togamy). There are a good many plants, however, which 

 have two forms' of flowers, viz., large, showy, nectar- 

 bearing, insect-pollinated ones, and small, inconspicuous, 

 self-pollinated ones, generally with a rudimentary perianth. 

 Flowers exhibiting this form of autogamy are said to be 

 cleistogamous. 



495. Examples are to be met with in some violets, wild 

 touch-me-nots, etc. ; early in the season these have large 

 flowers, which are pollinated by insects, but later only 

 small cleistogamous ones appear, and in some violets these 

 are subterranean. Without doubt it frequently happens 

 that the pollen of wind- and insect-pollinated flowers falls 

 upon their stigmas, resulting in accidental self-pollination ; 



