314 



THE EVOLUTION THEORY 



As is well known, the n\ale cells in the highest flowering plants 

 are not zoospernis but roundish cells, each of which, enclosed, 

 together with a sister-cell— the so-called 'vegetative' cell — in a thick 

 cellulose capsule constitutes a pollen-grain. The pollen-grains reach 

 the stigma, under which, buried deep within the 'ovule,' the female 

 sex-cell rests, enclosed in a long, sac-like structure called the 'embryo- 

 sac' (Fig. 82, A). Beside it {eiz) there lie several other cells, usually 

 seven in number, two of which, the so-called ' synergidne ' {f>y), have 

 their place at one end of the emlnyo-sac, just in front of the 

 ovum {ciz). Probably these give off a secretion which exercises an 



Fig 82. Fertilization in the Lily, Lilium martagon, after Guignard. A, the embryo- 

 sac before fertilization ; sy, synergidae ; eis, ovum ; op and u}), upper and lower 'polar 

 nuclei ' ; ap, antipodal cells. B, the upper part of the embryo-s;ic, into which the pollen- 

 tube {pschVj has peneti'ated with the male sex-nucleus (cj k'l and its centrosphere ; below 

 that is the ovum with its (also doubled) centrosphere (csph). C, remains of the pollen- 

 tube ipschl) ; the two sex-nuclei are closely apposed. Highly magnified. 



attractive (chemotactic) influence on the male fertilizing body (' the 

 pollen-tube '), and thus, so to speak, show it the way to the ovum. 



When a pollen-grain has reached the stigma it sends out a tube, 

 usually after a few hours, which penetrates into the soft tissue of the 

 style, and grows deep down into the interior of the ovule, ultimately 

 penetrating as far as the embryo-sac through a special little opening 

 in the covering of the ovule, the so-called 'micropyle' (Fig. H2B, pschl). 

 Its blunt end is now closely apposed to this, so that the true sperm- 

 nucleus {B, ^k), surrounded by some protoplasm, can leave the 

 pollen-tube and wander in among the cells of the embryo-sac. Later 

 on we shall see that two generative nuclei migrate from the pollen- 

 tube, but in the meantime we shall devote our attention only to one 



