POLLEN 345 



extension of the inline, makes its way. Of peculiar interest 

 are the grains of Zostera and of some related marine mono- 

 cotyledons. They are long and thread-like and coil round 

 the stigmas of the female flowers. The elongated form is 

 probably favourable to flotation, but is not found in all 

 genera. In plants with water-borne pollen the extine is 

 usually absent. 



The extine may be quite smooth or m.ay be sculptured 

 in various ways, bearing warts, ridges, spines, or delicate 

 bristles. Such roughened grains are found only in insect- 

 pollinated plants. The rough surface makes the grains 

 stick together or to other bodies, such as the insect's legs. 

 In some few cases, notably in certain Onagraceae, the grains 

 have several long delicate threads of viscid substance which 

 bind them together. In most of the Orchidaceae, and in 

 many Asclepiadaceae and Mimoseae, poUinia are formed. 

 In the first two families these may consist of the whole 

 mass of pollen of two neighbouring sporangia glued together ; 

 in the Mimoseae and in many orchids the number of pollinia 

 is greater. The pollinia are transported by insects, to which 

 they become attached by a variety of means. The advantage 

 of the poUinium is specially obvious in the orchids where 

 each ovary contains innumerable ovules. The deposition 

 of a single pair of pollinia on the stigma ensures the 

 fertilisation of large numbers of ovules. 



Germination of the pollen grain takes place by the protru- 

 sion of the intine through the extine and its growth into a 

 long slender pollen tube. In some cases, as in Malva, 

 several pollen tubes are formed, though only one is func- 

 tional. The conditions under which germination occurs 

 are very varied. In the angiosperms the pollen may be 

 capable of resting in the dry state for considerable periods 

 without losing its power of germination. Kerner relates 

 that the Arabs, who artificially pollinate the date palm, " put 

 aside some of the pollen from year to year, so that in the 

 possible event of the male flowers not developing, they may 

 ensure a crop of dates." He gives the extent of viability 

 for a number of other plants, ranging from 3 days in Hibiscus 



