Ii6 THE LIVING CYCADS 



apex, where the protoplasmic sheath is thickest, the 

 amoeboid movement is most conspicuous and may be so 

 rapid that it is more like spasmodic twitching. How 

 long the sperms might swim in the pollen tubes under 

 natural conditions it would be impossible to determine; 

 but under artificial conditions, in a sugar solution, the 

 movements have continued for five hours. 



After the sperms begin to move there is a rapid 

 increase in the turgidity of the tube, which sooner or 

 later ruptures at or near the exine of the pollen grain. 

 Most of the starch and liquid contents of the tube escape 

 with a spurt, unless one of the sperms is immediately 

 drawn into the opening. The first sperm may escape 

 in two or three seconds, but the other may be half a 

 minute in getting out, probably because there is not 

 so much pressure behind it. The rupture is often not 

 more than 50 /^ in diameter, while the average sperm is 

 four times as broad. But however much the sperm may 

 be constricted in getting out, it promptly regains its 

 form and begins to swim. 



Efforts to keep the sperms alive after their escape 

 from the pollen tube were not very successful. In weak 

 sugar solutions they immediately break to pieces, almost 

 explode; in a 10 per cent solution they simply die; in a 

 12 or 15 per cent solution they live a few minutes; in a 

 20 to 25 per cent solution they live a little longer, but 

 this is about the maximum, for solutions stronger than 

 these were less and less satisfactory. Of course the 

 behavior under natural conditions could not be deter- 

 mined. 



When the pollen tubes begin to discharge, the 

 archegonial chamber is moist but contains no free liquid. 



