Classification of Plants 



237 



developed the pollen tube mentioned on p. 159. When the 

 pollen tube reaches its destination in an angiospermous ovule 

 (Fig. 210) it discharges two nuclei which it has brought the 

 entire journey. One serves as a sperm nucleus. For a long 

 time the fate of the other was a mystery. It was finally found 

 in many angiosperms to fuse with the secondary nucleus, thus 

 taking part in the formation of the endosperm. 



Another remarkable discovery revealed the fact that in 

 some Gymnosperms of the Cycadales and in Ginkgo (the 

 Maiden-hair tree) the sperm cells have not quite lost the feat 

 of swimming, but after the pollen tube has discharged them 

 in a depression, the archegonial chamber, the sperm cells 



Fig. 211. — Encephalartos villosuSj'L.ehm. Staminate cone, left side. Ovu- 

 late cone, right side. " Seedling perhaps two years old " (Miss Pegler), 



swim in a liquid discharged from the pollen tube and enter 

 the archegonial necks by their own activity. This fact is of 

 especial interest in -showing the relationship between the higher 

 and the 'lower plants. 



