STUDIES OF SEED-PLANTS 



197 



The first step in this transfer is the placing of the grains upon 

 the stigma or upper end of the pistil. This transferring of 

 pollen-grains, called pollination, is a purely mechanical 

 process, sometimes done by insects, sometimes by wind, 

 sometimes the pollen falls from anthers to stigma, and often 

 gardeners make the transfer by means of a small brush 

 which is first touched to anthers and then to stigmas of 

 flowers. A large number of flowers have their sepals, petals, 

 anthers, stamens, and pistils modified to fit them for the 

 visits of insects, or to provide for pol- 

 lination by other means; and such 

 modifications will be considered later. 



Fertilization. When pollination has 

 occurred, that is, when pollen-grains 

 have lodged on the stigma of the pistil, 

 the wall of the pollen-grain grows out 

 into a tube (pollen-tube) and this grows 

 down between the cells composing the 

 style and ovary until it reaches the 

 egg-cell in the ovule (see Fig. 61). One 

 pollen-cell is required for each egg-cell, 

 so that in a flower with a dozen egg- 

 cells there must be a dozen pollen-tubes 

 from as many pollen-grains growing 

 down the style. Many more pollen- 

 grains may be on the stigma and each 

 may grow a pollen-tube, but only one 

 is needed for each egg-cell. 



When the pollen-tube has nearly 

 reached the egg-cell, as shown in Fig. 61, 

 the end of the tube opens and some of the contained proto- 

 plasm and a nucleus from the pollen grain slips down the 

 tube and goes into the egg-cell. This mass which enters the 

 egg-cell is the fertilizing cell or sperm-cell, and its nucleus is 

 the sperm-nucleus. The sperm-nucleus soon unites with the 



FIG. 61. Longitudinal 

 section of ovule of 

 pine, with two egg- 

 cells (n, o) , each with 

 a nucleus, p, pollen- 

 grains ; t, pollen-tube ; 

 e, endosperm ; i, in- 

 tegument. The en- 

 tire ovule forms a 

 seed with one embryo, 

 one of the egg-cells 

 not developing. (From 

 Strasburger.) 



