16 FERTILIZATION. [chap. 



SO that at hngth it protrudes, and, like an excessively minute 

 root-fibre, penetrates the substance of the stigma, and passes 

 down through the short style until it reaches the cavity 

 of the ovary. As the changes of which we speak can only 

 be observed under a considerable magnifying power, we 

 shall explain them more clearly by reference to the cut. Fig. 

 12a, which represents some grains of pollen which have 

 developed tubes reaching into the ovary. 



4. The ovary contains several minute seed-buds, the 

 ovules; which ovules in the Orange are inverted {ana- 

 tropous). Each ovule consists of a central cone, called 

 the micleus of the ovule, around which central cone is a 

 layer of cells forming the coat of the ovule. This cellular 

 coat grows up around the nucleus, and closes over it 

 excepting at the top, where a very minute aperture through 

 the coat is always left. This aperture is called the micro- 

 pyle. Owing to the ovules of the Orange being anatropous, 

 the micropyle is brought close to their point of attachment 

 ihilum) ; and as they are pendulous more or less, it is 

 directed upwards. 



By the time that the pollen-tube has reached the cavity 

 of the ovary, certain important changes have taken place 

 m the cells which form the nucleus of each ovule. One 

 cell has enlarged greatly at the expense of its neighbours, so 

 as to occupy a considerable part of the nucleus. This 

 enlarged cell is called the emhryo-sac, because within it we 

 find the embryo to originate. But this is not until aftei 

 the pollen-tube has reached the micropyle of the ovule and 

 actually penetrated to the upper end of the embryo-sac, 

 against which it becomes closely applied. Presently, after 

 this contact of pollen-tube and embryo-sac, a membranous 

 cell-wall forms around a condensed segregated portion of 

 the upper part of the embryo-sac, independently of the 



