192 TBAJSTSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



less elastic than the side walls of the tube is stretched by 

 this internal pressure and the stretching being rendered 

 permanent by growth, the pollen tube grows in length. 

 It may happen that the soft ductile region is not terminal 

 but remains somewhat lateral in position. In such cases 

 the pollen tube grows spirally and the closeness of the 

 spiral is determined by the amount of deviation of the 

 softened apical region from the apex of the tube. If the 

 terminal softer apical region divides into two more laterally 

 placed areas the pollen tube will fork at this spot. The 

 branching of the pollen tube, which is of rare occurrence 

 and which apparently takes place only under abnormal 

 conditions, seems, except in certain cases to be mentioned 

 later, to be always of apical origin and is hence dicho- 

 tomous though it may happen that one of the branches 

 becomes larger than the other and continues the growth 

 and direction of the unbranched tube. 



In certain cases the growth of the tube having been 

 checked owing to a diminution of turgidity the wall of the 

 terminal apical portion being no longer continually 

 stretched becomes thicker, shews faint lamellation and is 

 evidently softer and contains more water than the older 

 part of the tube. It sometimes happens that not only 

 the wall of the extreme apex but also of the terminal 

 portion of the pollen tube remains soft and ductile, and in 

 such cases any increase in the internal turgidity causes a 

 swelling or bulb like dilatation to be formed at the apex 

 of the tube. These apical dilatations are found most 

 abundantly in old cultures, though they may be formed 

 as soon as the first protrusion of the endosporium takes 

 place. The formation of these swellings seems to be the 

 final stage in the life history of the pollen tube for in no 

 case has a commencement of the growth and elongation 

 of the tube been seen after they have once been formed, 



