98 



remove the pollen masses, and Waetcher, who seems not to 

 have been acquainted with Sprengel's work, stated that in- 

 sects are necessary for the pollination of orchids. Orchids 

 have been the objects of a large number of interesting pa- 

 pers. The order is represented by more than three thousand 

 species. Few of our native species compare in beauty with 

 those of the tropics. In habit they are extremely diverse. 

 Hermann Mueller says: "1 do not doubt that orchids owe 

 their extraodinarily perfect adaptation to ])articular insects 

 not only to the tendency of the parts of their flowers to vari- 

 ation, but also to the separation in time of the two stages in 

 the act of impregnation. The extremely complete adapta- 

 tion to cross-fertilization have in their turn resulted, in many 

 cases, in the flowers becoming absolutely sterile to their own 

 pollen." 



It may take days, weeks, or even months for the pollen- 

 tube to reach the ovule and cause impregnation. It may be 

 interesting to observe that the period of flowering as in many 

 cases extended over a considerable length of time. 



Orchids differ in regard to their capacity for self fertil- 

 ization; some have cleistogamous flowers, as in Cattleya 

 and Epidendrum'-, some are self-pollinated, or at least oc- 

 casionally so, frequently in Neottia nidus-avis; others are 

 never self pollinated although when their own pollen is ap- 

 plied artificially it is fertile, while some are absolutely ster- 

 ile to their own pollen though fertile to pollen from another 

 plant of the same species or to the pollen from a plant of 

 another species. In some cases the pollen of a plant acts as 

 a poison when applied to its own stigma. 



In Older to fully understand the mechanism for the 

 pollination it will be necessary to give the parts of an orchid 

 flower. The perianth consists of six parts in two sets; the 



