306 



MORPHOLOG Y. 



gates by the formation of a tube, forming a sac, known as the pollen tube 

 It is either simple or branched. It grows down into the tissue of the nu- 

 cellus, and at a stage represented in fig. 358, winter overtakes it and it 

 rests. At this time the central cell has divided into two cells, and the 

 vegetative nucleus is in the pollen tube. 



622. The endosperm. In the following spring growth of all these parts 



Fig. 359- 



Section of nucellus and endosperm of white pine. The inner layer of cells of 

 the integument shown just outside of nucellus; arch, archegonium; en, egg nu- 

 cleus. In the nucellar cap are shown three pollen tubes, vn, vegetative nucleus 

 or tube nucleus; stc, stalk cell; spn, sperm nuclei, the larger one in advance is 

 the one which unites with the egg nucleus. The archegonia are in the endosperm 

 or female gametophyte. (After Ferguson.) 



continues. The nuclei in the macrospore divide to form more, and event- 

 ually cell walls are formed between them making a distinct tissue, known 



