POLLEN. 101 



with this oil, I ascertained that in about 400 out of 520 species investigated by 

 ine the outer surface was overlaid with oil. The layer is so thin that with dry 

 pollen-grains it is not visible, but when they are placed in water, the coating is 

 resolved into a number of minute, strongly -refringent droplets, which adhere to the 

 swollen pollen-grains like tiny beads. There is no doubt that this coat consists of 

 a fatty oil, since it is soluble in alcohol and olive-oil, and with osmic acid it turns 

 dark-coloured and becomes congealed. 



More rarely are pollen-grains found with masses of a sticky, structureless 

 substance adhering to them. This substance does not form droplets with water, nor 

 does it dissolve in alcohol and olive -oil. It may be termed Viscin, from the 

 similarity which it presents tc the bird-lime obtained from the berries of the 



1 (k ^^-# 



Fig. 219.— Pollen-grains and pollen-tetrads united by threads of viscin. 

 K ^ Rhododendron hirsutum. ^ CEnothera biennis. * Epilobium angustifoliiim. ixS; --*x50. 



Mistletoe ( YiscuTn). Such a viscin is met with on the surface of the pollen-grains 

 of Fuclisia, ClarJda, Circcea, Gaura, Godetia, (Enothera, Epilobium — indeed, 

 throughout Onagracege and in Azaleas, Rhododendrons, Orchids, and Asclepiads. 

 It is very sticky, and on the slightest touch can be drawn out into delicate threads. 

 The contents of the anthers, as they escape, in the Evening Primrose (GEnothera) 

 and Willow-herb {Epilubium angustifoliura) resemble fringes and tattered ribbons, 

 or a broken net hanging from the adjacent anthers. Under the microscope this sub- 

 stance is seen to consist of pollen-grains, joined together by the sticky strings of viscin 

 (fig. 219 ^ and 219 *). The phenomenon is even more striking in the numerous species 

 of Rhododendron. In Rhododendron hirsutum all the pollen-tetrads of an anthei-- 

 cavity are held together by a mass of sticky viscin. The anther dehisces by two 

 terminal pores, and from these the pollen-tetrads ooze out to some extent. If the 

 sticky mass be touched with a bristle it adheres, and the whole contents of the 

 anther can be readily withdrawn (fig. 219^). Its appearance under the microscope 

 is shown in fig. 219 ^. In many species, as for instance in the elegant Rhododendron 

 Chamcecistus of the Northern Limestone Alps, and in the large-flowered Himalayan 



