302 INTRODUCTION TO BOTANY 



During the growth of the pollen tube the single cell which 

 at first was in the pollen grain has developed until it now con- 

 sists of three cells, which are carried near the tip of the tube. 

 Two of these cells are male cells and can serve as sperms, al- 

 though they are not ciliated. The other cell is a nutritive cell, 

 which goes forward with the tip of the pollen tube and is an 

 important factor in its growth. These cells all pass out of the 

 tube through an opening in its tip when it has reached the 

 embryo sac. One of the male cells unites with the egg cell, 

 and in many cases, perhaps generally, the other male cell passes 

 to the center of the sac and unites with the endosperm cell. 

 The first union of cells is like that of the gymnosperms, but 

 the second does not occur except in the angiosperms. This 

 union of both the male cells with other cells in the same em- 

 bryo sac is known as double fertilization. After fertilization 

 the egg grows into a new plant, and the endosperm cell grows 

 into the endosperm, or food material, that is stored in the seed 

 around the embryo plant. Double fertilization gives to the seed 

 outside the embryo, as well as to the embryo itself, the charac- 

 ters of both parent plants. In this way, when plants such as 

 different kinds of corn are crossed, the ear thereby produced 

 will have some grains showing the characters of both parents. 



287. The new plant. The way in which the fertilized egg 

 grows into the new plant is striking. The egg cell first divides 

 and redivides in such a way as to form a chain of cells. One 

 end of this chain may become elongated and, as the suspensor, 

 may attach the developing embryo to the wall of the embryo 

 sac. The cells at the other end of the chain are the ones that" 

 produce the parts of the new plant the stem tip, leaves, and 

 root tip (fig. 232). The embryo in some kinds of angiosperms 

 has but one leaf, which grows at the end, and the young stem 

 is at the side of the embryo. These are the angiosperms with 

 one seed leaf, or the monocotyledons. In other cases the embryo 

 may have two (or even more) seed leaves, and the stem is at 

 the tip and the leaves are on the sides of the embryo. These 

 are the dicotyledons, or plants with two seed leaves. 



