240 COMPENDIUM OF GENERAL BOTANY. 



Furthermore, it is clear that since the visit of insects depends upon 

 the existing plan of organization it also guarantees tlie greatest 

 success. 



2. Dicliny. It is either monoecious^ that is, male and female 

 flowers occur upon the same plant (example : Zea 2fays), or 

 dioecious, if the sexes occur upon different plants (examples : Salix, 

 Conifer oe). 



3. Heterostyly. In plants of the same species (examples : 

 PrimulaceoB, Lythrum Salicaria) there may be two or three sets 

 of stamens differing in length {dimorj)hism, trimorphisni) ; corre- 

 sponding to these stamens, the pistils are also of different lengths. 

 The following is the principle underlying this arrangement. That 

 part of the body of the insect which comes in contact with tlie 

 stamens of the lengtli a of one flower also comes in contact with 

 the stigma of the same length of another flower. The above 

 method of pollination produces the best results, as has been verified 

 by control experiments. It has also been observed that in flowers 

 with elements of unequal length the female flowers are more or 

 less sterile. In-and-in breeding (Inzucht) is tlie term applied to 

 that form of reproduction which occurs in tlie same plant-forms, in 

 distinction to hybridization (crossing). We also know that the 

 most common form of crossing is by two different individuals of 

 the same species (cross- fertilization in the na; rower sense), the 

 special organization of which we have just learned to know. There 

 are, however, certain plants with flowers especially adapted for self- 

 jpollination (Hermann Muller made a special study of this adapta- 

 tion). Many land-plants open their flowers only partially or not at 

 all on rainy days ( Veronica hedercefolia, Drosera rotundifolia) ; 

 the submerged flowers of water-plants also remain closed, and 

 pollination takes place in the small air-space between the floi-al 

 coverings.' Example: Ranunculus aquatilis. %Mch cleistogamons 

 flowers develop much less pollen than the chasmogamous floweis, 

 which are dependent upon wind and insects for pollination. The 

 cleistogamous flower of Viola nana forms about 100 poUen-grains, 

 while an entomophilous flower of Leontodon forms about 243,600. 



ScHENK, Biologie der "Wassergewachse. 



