0BSEBVATI0NS UPON THE POLLEN TUBE. 195 



which the disturbing influence due to the presence of 

 foreign micro-organisms may be considerably reduced. 

 Fungal hyphse and Yeast cells being absent whilst 

 Bacteria are for a time at least relatively few in number. 

 It is impossible to obtain any fairly large quantity of 

 pollen which shall at the same time be entirely free from 

 the spores of Bacteria and any attempt to destroy these 

 would necessarily involve the destruction of the pollen 

 grains also. The spores thus introduced though perhaps 

 at first but few in number soon by their rapid development 

 and multiplication spoil the cultures, but for the first two 

 or three days and especially if the tubes are exposed to 

 bright sunlight the numbers present are fairly low. 



Difficult as it is to obtain pure tube cultures of pollen 

 and to keep such free from the disturbing influence of 

 Bacteria, etc., yet this is the only way by which a pro- 

 longed experimental culture can be made ; a sample of 

 the developing tubes being examined from time to time. 

 Here of course chances of error owing to the necessary 

 manipulation are introduced and the sample may not, 

 unless two or three are taken, accurately represent the 

 condition of the bulk of the culture. 



Unringed slide cultures on the other hand rapidly 

 evaporate and if ringed with vaseline to prevent evapora- 

 tion, the supply of oxygen which the enclosed fluid 

 contains is soon used up and the growth of the pollen 

 tubes ceases. The rapid stoppage of growth which thus 

 occurs is due to the cessation or diminution of the tur- 

 gidity of the pollen tube, probably owing to a diminished 

 percentage of osmotically active substances, organic acids, 

 etc., in the cell sap and this again is due to the deficiency 

 of oxygen. 



When attempting to inhibit the development of Bacteria 

 by exposing tube cultures of Narcissus pollen to the action 



