THE FLOWER 231 



fairly frequently, because the number of pollen grains 

 which penetrate into the ovary is generally considerable. 

 Observations have, however, been made under the 

 microscope which point to the fact that pollen-tubes 

 are definitely attracted towards the tissue of the stigma 

 or style, if it lies sufficiently near to them. This tissue 

 seems to attract them even after it has been killed by 

 boihng. 



Meanwhile events are occurring in the nucellus. In the 

 upper part of the embryo-sac three little cells are formed 

 out of its protoplasm. These cells have no cellulose 

 wall, and are therefore only spherical lumps of thick 

 protoplasm each with a nucleus. One of them is called 

 the egg-cell or ovum, because it is the actual origin of 

 the future embryo of the plant. This is therefore the 

 primary cell we were looking for ; from which, as we 

 mentioned, every plant arises, be it a spore-bearing plant 

 or a seed-plant.^ The egg-cell is enclosed in the upper- 

 most part of the embryo-sac so that the tip of the pollen- 

 tube, on reaching the nucellus down the canal of the 

 ovule, comes into close contact with the egg-cell (fig. 65, 

 at the bottom, on the left-hand side). 



Recent investigations have proved that the act of 

 fertilisation really consists in the passage of a nucleus, 

 from the end of the pollen-tube, through the softened 

 and dissolved wall of the tube into the embryo-sac (the 

 wall of which also becomes softened or dissolved) , and its 

 fusion there with the nucleus of the ovum. It is curious 

 to notice that half of the chromatin substance involved 

 in this process, i.e. half the total number of rods, belong 

 to the male and half to the female nuclei respectively, so 

 that the first nucleus of the embryo, formed by their 

 fusion, contains chromatin from both paternal and 

 maternal organisms. This fact explains very obviously 

 why the properties of the parent plants are blended in 

 their posterity. We shall soon see that in the flowerless 

 1 See chapter i. 



