POLLEN GRAIN OF ELDER 163 



The first two mitoses in tlie embryo sac of the lily show that it is a mega- 

 spore mother cell in this plant (as also in related types), which later contains 

 the female gametophyte. The nucellus is therefore a megasporangium and 

 the carpel a megasporophyll. 



E. Fertilization and double fertilization. Stages showing the nuclear fusions 

 of fertilization and double fertilization are, of course, not common, but 

 one preparation will serve for a demonstration of the processes. The 

 pollen tube brings two sperm nuclei into the embryo sac. One of these 

 unites with the egg nucleus, fertilizing it, and the other unites with the 

 two polar nuclei, forming a triple fusion which results in the large 

 endosperm nucleus in the center of the sac (see Principles, Fig. 307). 



F. The development of the^ embryo and endosperm. The fertilized egg 

 nucleus with surrounding protoplasm heoomes the fertilized egg, and, 

 forming a cell wall about itself, proceeds to develop the embryo sporo- 

 phyte at the micropylar end of the embryo sac. The endosperm nucleus 

 gives rise through successive divisions to a very large number of nuclei, 

 which become distributed in the layer of protoplasm which lines the em- 

 bryo sac (see Principles, Fig. 308). Cell walls are later formed between 

 these nuclei, and the embryo sac becomes filled with a delicate tissue 

 called the endosperm, in which the developing embryo lies imbedded. 



It is important to note that the endosperm of all the angiosperms is a 

 development following fertilization and therefore not strictly comparable 

 to the endosperm of gymnosperms, as illustrated in the pine, which is 

 formed before fertilization and is therefore strictly gametophytic in charac- 

 ter without the complication of double fertilization on the union of polar 

 nuclei. The group of three antipodal nuclei in the embryo sac of the 

 angiosperms may represent the endosperm of gymnosperms, but this is not 

 fully established. The female gametophyte of the angiosperms is much 

 simpler than that of the gymnosperms, having only eight nuclei, one egg, and 

 no clearly defined archegonium. The male gametophyte of the angiosperms 

 is likewise simpler, containing only three nuclei, two sperm nuclei, and the 

 tube nucleus. 



142. The pollen grain, or microspore of the elder. Some pollen grains at 

 maturity contain, the male gametophytes much further advanced than is 

 shown in those of the lily. This is well illustrated in the elder (Sambucus). 

 Unopened stamens should be fixed and preserved in alcohol. The anthers of 

 such may be teased apart in water, allowing the pollen grains to escape. 

 When stained with eosin these grains will be seen to contain three nuclei, a 

 tube nucleus lying freely in the center of the cell, and two sperm nuclei 

 somewhat at one side, surrounded by denser protoplasm, forming two male 

 cells (see Principles, Fig. 305). The nuclear divisions are completed in this 

 male gametophyte at the time the pollen is shed, and the only further 



