OBSERVATIONS UPON THE POLLEN TUBE. 197 



sky becoming overcast with cloulds. It was only after 

 dozens of experiments had been made during the summer 

 months that satisfactory results were obtained. The 

 experiments were performed with pollen of Narcissus, 

 Cheiranthus and Lathy rus. In each case after a certain 

 length of time samples from each culture were taken and 

 the number of pollen grains, and the number and average 

 length of the pollen tubes in each estimated and compared. 



The following are illustrative examples of the results 

 obtained : 



Narcissus examined after 14 hours exposure. 



(1) In darkness. Average length of tubes 4 to 6 times 



diameter of pollen grain. 



(2) In diffuse daylight. Average length of tubes 6 to 



10 times diameter of pollen grain. 



(3) In bright sunlight. No tubes formed. 



In a few cases in (3) the commencing protrusion of the 

 pollen tube was seen but not more than this. Examined 

 at the end of the second day (i.e., after exposure for two 

 days and one night) a number of tubes were seen to have 

 been formed. These averaged only half the length of the 

 pollen tubes of (2) and compared with (2) 40 per cent, less 

 tubes were formed to the same number of pollen grains. 

 The major part of this growth and germination takes 

 place during the period of darkness necessarily interposed 

 between the two periods of exposure to sunlight. The 

 same experiments with pollen of the wallflower gave 

 similar results. Thus exposed to sunlight but few 

 germinated whilst in diffuse daylight and in darkness 

 a larger percentage germinated, the average lengths 

 of the pollen tubes being respectively from 2 to 4, from 

 8 to 10, and from 7 to 9 times the diameter of the pollen 

 grains when examined after being in the culture solution 

 for a night and a day. Pollen of Lathyrus odoratus after 

 10 hours, during 5 of which the sun was shining : — 



