FERTILIZATION AND THE ORIGIN OF SEEDS 209 



ing from the grains, and into these the contents pass as 

 a living stream (Fig. 142). 



When pollen-grains are placed on the stigma of a pistil, 

 growth of this kind occurs. The pollen-tubes grow down- 

 wards through the style, enter the cavity of the ovary, and 

 reach the micropyle of the ovule. Into this the tube passes, 

 and the essential part of its contents enters the embryo- 

 sac and there accomplishes the process of fertilization by 

 fusing with the nucleus of the egg-cell. It is not until such 

 fertilization has taken place that the ovule develops into 

 a seed ; for if the stamens are removed from a flower before 



Fig. 142. Germinating Pollen-Grains. n, nucleus ; 

 p, protoplasm ; P.g, pollen-grain ; P.t, pollen-tube. 



the anthers are ripe and the flower is covered so that no 

 insect can carry pollen to it, the pistil is unable to set ripe 

 seed, although it may enlarge considerably and in some 

 cases become fleshy. 



Changes produced by fertilization. After the egg-cell is 

 fertilized, the rest of the embryo-sac becomes filled with 

 tissue called endosperm. In the Pea and the Bean the 

 fertilized egg-cell grows at the expense of the endosperm, 

 develops into an embryo, and at the same time the nucellus 

 is absorbed. Thus, when the seed is ripe it contains no 

 endosperm. 



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