THE ANGIOSPERMAE 



1375 



serves to separate each massula from its neighbours. All the massulae are 

 loosely coherent and form a compound poUinium. The pollinia of Asclepia- 

 daceae are similarly formed of coherent massulae. 



Fig. 1278. — Pollen organization in Orchidaceae. The 

 dotted lines represent permeable walls, the con- 

 tinuous lines impermeable walls. The former 

 permit synchronization of nuclear divisions. 

 {After Barber.) 



The cohesion of massulae in these families and the looser cohesion of 

 single grains in many other cases is due to a mucilaginous substance called 

 viscin, from its resemblance to the sticky " bird-lime " in the berries of 

 Viscum. It is of complex but uncertain composition. Besides Orchidaceae 

 it is found particularly in the Ericaceae and Onagraceae. When the anthers 

 in a member of these families dehisce, the pollen trails out in ragged 

 streamers which adhere to any object, even at the lightest touch. The 

 anthers of Ericaceae open by pores and if the escaping pollen is touched the 

 whole contents of the anther may be pulled out at once. Insects contacting 

 the viscin will therefore carry off an abundance of pollen to the next flow^er 

 visited. 



Of systematic interest from the point of view of the relationships 

 betw^een Dicotyledons and Monocotyledons are the observations of Bailey 

 and Xast on the pollen of Ranales (in Engler's sense). They showed that 



