FERTILIZATION. 171 



take place in the structure of the ovule as the embryo forms 

 in its interior. 



In some instances, though rarely, all the parts of the ovule 

 are visible in the seed ; but in general these parts either dis- 

 appear altogether, as the embryonic mass increases in bulk, or 

 are very materially altered. In many Dicotyledons, the em- 

 bryo as it develops absorbs into itself not only the embryo 

 sac but the tissue which forms the nucleus, so that the seed 

 at its maturity contains nothing but an embryo of which the 

 cotyledons are thick and fleshy, by the amount of nutritious 

 matter which they have absorbed, and the integuments of the 

 ovule, the primine and secundine, which form its general 

 covering. This is the case for example in the Leguminous 

 family. The pea (Pisum) is a good illustration. 



But in other Dicotyledonous plants, and in all Monocotyle- 

 dons, the nutriment which the ovule contained in its interior 

 is unabsorbed into the embryo, which does not increase much 

 in bulk, and encroaches very slightly on the cells of the nu- 

 cleus. These cells therefore become filled with a deposit of 

 solid matter termed albumen, in the midst of which the embryo 

 is embedded. 



Seeds in which the embryo occupies the entire seed are 

 called ex-albuminous (ex without), as the Compositse, Cruci- 

 ferae, and Leguminosae, whilst others having separate albumen 

 are albuminous. The larger the quantity of albumen in the 

 seed, the smaller the embryo. 



Soon after fertilization, the pollen tube withers from above 

 downwards, the foramen or micropyle of the ovule closes, and 

 when the embryo is fully developed within it, the ovule 

 becomes the seed and the ovary the fruit. 



The changes which manifest themselves in the flower and in 



