PROTECTION OF POLLEN 351 



too, that the nocturnal closure protects against over-cooling, 

 but of such an effect we have no good evidence. Some 

 flovi^ers — the tobacco, the evening primrose, the night- 

 scented stock — are closed by day. They are pollinated by 

 night-flying moths and are open, and most strongly fragrant, 

 when these are abroad. The significance of the day closure 

 we do not know, nor has its causation been cleared up. 



It is clear that some types of floral architecture, and those 

 movements which result in night closure, must protect the 

 pollen, but the relation of structure and movement to the 

 necessities of a particular plant does not seem to be a very 

 close one. It is significant that no case is known in which 

 the movement is controlled by change in atmospheric 

 humidity. Hansgirg (1904), indeed, refers a large number 

 of floral movements to moisture changes, but in no case 

 with sufficient proof. As a number of the cases given by 

 him, e.g. the daisy, the scarlet pimpernel, the anemone, are 

 certainly related to temperature or light his entire work must 

 be taken as requiring revision. 



Protection o£ Pollen and Sensitiveness to Damage. — 

 Lidfors (1896) has correlated the sensitiveness of the pollen 

 to damage by rain with the degree of protection existing in 

 the flower. He examined representatives of 80 families ; 

 55 of these included species where unprotected pollen was 

 resistant, while 23 had species with unprotected sensitive 

 pollen ; 23 families had species vdth protected pollen which 

 was damaged by water, while 6 had species with protected 

 and resistant pollen. He notes other features which are 

 seen where unprotected pollen is sensitive ; thus in the 

 grasses the pollen germinates very quickly and is produced 

 in great preponderance over the ovules, though this is, of 

 course, related to the mode of pollination. He also gives 

 some interesting contrasts between related species. In 

 Rumex the pollen is resistant and quite unprotected ; in 

 the closely related Polygonum the pollen is protected and is 

 sensitive. His conclusion is that on the whole unprotected 

 pollen is resistant, sensitive pollen is protected. 



Hansgirg (1904), on the other hand, cites a large number 



