198 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



(a) Direct sunlight. Length of tubes 3 to 4 times 



diameter of pollen grain. About 10 p.c. of pollen 

 grains germinated. 



(b) Shaded from sunlight. Length of tubes 6 to 8 times 



diameter of pollen grain. 15 p.c. germinated. 



(c) In darkness. Length of tabes 6 to 8 times diameter 



of pollen grain. 20 p.c. germinated. 

 After 36 hours, 12 hours full sunlight, 14 hours daylight 

 but no sun, 10 hours darkness and twilight. 



(a) Direct sunlight. Average length of tubes 7 to 10 



times diameter of grain. 



(b) Shaded from light. Average length of tubes 15 to 



18 times diameter of grain. 



(c) In darkness. Average length of tubes 14 to 16 



times diameter of grain. 



In these tubes exposed to diffuse daylight the tempera- 

 ture is distinctly lower than in those exposed to the sun, 

 and hence the growth of the pollen tubes, in such cultures, 

 is not so rapid as it would otherwise be; but on the 

 other hand the excessive development of Bacteria, etc., 

 checking the growth of the pollen tubes is less marked in 

 the cultures exposed to diffuse daylight than it is in the 

 darkened tubes. These two antagonistic influences partly 

 neutralize each other but the influence of the lower 

 temperature in retarding the growth of the tubes is much 

 more marked and produces a direct and immediate effect. 

 Hence in (b) the growth of the tubes would be greater 

 than in (c) were the temperatures equal in the two cases. 



These results show that direct sunlight retards the 

 growth of the pollen tube and may inhibit the germination 

 of the pollen grain whilst exposed to weak diffuse day- 

 light. The rapidity with which the pollen tube elongates 

 is slightly greater than it is in absolute darkness. In 

 nature, a pollen tube is neither exposed to direct sunlight 



