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 some 

 of the most wonderful modifica- 

 tions of the parts of the flower 

 to secure cross pollination. Fig- 

 ure 106 represents, in part, the 

 construction of the flower of 

 Caiasetum tridcntatuni, 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 

 J.JQ jq5_ body c, known as the column, is 



A, longitudinal diagrams of the COmpOSed of the pistil COnfluent 

 ^o^^r oi Catasetum tridentatum. ^.^ ^^ j^ j g^^^g^. The 



c, the column ; h, the sensitive ° 



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



^. proiection of the poUinium and , it i_ r i • i 



viscid disk after the stimulation tWO pollen SaCS, each of whlch 



of the spur by the touch of an contains a maSS of pollcn, p. 



insect. After KERNER. r^, ,, 



Ihe 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, Ji, 

 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, 



