190 



Introduction to Botany. 



bees transfer pollen from the younger flowers to the 



stigmas of the older (4 and 5). 



139. Cross Pollination of Or- 

 chids. In orchids we find so'me 

 of the most wonderful modifica- 

 tions of the parts of the flower 

 to secure cross pollination. Fig- 

 ure 1 06 represents, in part, the 

 construction of the flower of 

 Catasetum tridentatum, a South 

 American orchid. In this flower, 

 as in orchids in general, there 

 are three colored sepals and the 

 same number of colored petals, 

 one of the petals serving as a 

 landing place for insects, and 

 being prolonged into a spur be- 

 low for the conservation of the 

 nectar. The irregular central 

 body c, known as the column, is 



A, longitudinal diagrams of the composed of the pistil Confluent 

 flower of Catasetum tridentatum. ,! ,1 i T^, 



,, the column; h, the sensitive Wlth the Sln g le Stamen. The 



spur; /, poiiinium; d, viscid disk, upper part of the column bears 



B, projection of the pollinium and , -,-, , f i i 



viscid disk after the stimulation tw P llen SaCS > ea ch of which 



of the spur by the touch of an contains a mass of pollen, /. 



insect. After K.ERNER. _,, ,, 



The pollen masses are con- 

 nected by means of an elastic band with a body termed 

 the viscid disk d, which is really a modified portion of the 

 stigmatic part of the column. Running down from the 

 central portion of the column are two slender horns, //, 

 standing in the way of insects' which would gather the 

 nectar or eat the fleshy parts of the flower/ 



As soon as an insect's head touches one of these horns, 



FIG. 106. 



