OBSEBVATIONS UPON THE POLLEN TUBE. 199 



nor is it in absolute darkness, the stigma being as a 

 general rule shaded from the sun whilst through the 

 partially transparent tissue of the style into which the 

 pollen tube burrows a certain amount of light can 

 penetrate. 



In aerial plants light of any intensity retards growth 

 because it promotes transpiration and hence lessens that 

 turgidity of the growing organs and regions which is so 

 necessary a factor in the continuance of growth. In the 

 case of a pollen tube, however, which is growing immersed 

 in a nutrient solution it is evident that light can exercise 

 no such influence. The effect of direct sunlight in 

 retarding growth is due to a direct action upon the 

 protoplasm of the pollen tube, causing it to assume a 

 condition of "immotility" and hence causing a cessation 

 of turgidity and stoppage of growth. Darkness exercises 

 a similar but very much weaker influence. The motility 

 of the protoplasm of the pollen tube and the turgidity of 

 the latter are most pronounced when exposed to weak 

 diffuse daylight. The four factors which influence the 

 rapidity of growth of the pollen tube are ; the nature and 

 strength of the nutrient medium, the relative amount of 

 oxygen present, the intensity of the light to which the 

 culture is exposed and the temperature at which it is kept, 

 the optimum being between 20° and 30° C. 



When examining two and three days old cultures of 

 Narcissus pollen it was noticed that in some cases the 

 pollen tubes had become septate. The septa were in the 

 form of rather thick transparent transverse partitions and 

 were few in number, there being rarely more than four or 

 five in one pollen tube. At first sight the appearance of 

 these septa seems to indicate that the pollen tube had in 

 such cases undergone cellular division and become a 

 multicellular filament, for the septa may not only divide 



