REPRODUCTIVE CELLS. 127 



c. Fertilisation and Subsequent Changes The matured 



microspore lying on the- stigma of the pistil of an Angiospermous 

 plant now undergoes the following further changes : — First, the 

 large vegetative cell of the microspore (see p. 121) grows 

 down into the conducting tissue (the central loose tissue) of the 

 style in the form of an elongated cell, the pollen-tube. This 

 process is brought about by the action of enzymes in the tube, 

 which dissolve the cellulose walls of the cells of the conducting 

 tissue; a further action of these enzymes being the conversion 

 of the starch in the cells into dextrins and sugar, which furnish 

 nutriment to the tube during its progress. Having reached the 

 cavity of the ovary, the pollen-tube is attracted towards the 

 micropyle of an ovule, the tip penetrates the micropyle, and 

 grows through the nucellar tissue to the upper pole of the 

 embryo-sac (the attraction of the tube being probably of the 

 nature of positive chemotaxis). The next change which occurs 

 is the passage of the nucleus of the generative cell (see p. 121) 

 to the apex of the pollen-tube, where it divides into two. One 

 of the resulting nuclei then passes between the synergidae, which 

 are situated at the upper pole of the embryo-sac, and, having 

 reached the egg-cell or oosphere, penetrates into this cell, 

 and, after a short time, fuses with the egg-cell nucleus, the 

 cytoplasm also fusing with that of the egg-cell. This pro- 

 cess completes the fertilisation of the oosphere, which now 

 becomes the oospore. The other nucleus resulting from the 

 division of the generative nucleus also passes between the 

 synergidae, and fuses with the definitive nucleus, which now 

 becomes the endosperm nucleus. Eound this latter the remain- 

 ing cytoplasm of the embryo-sac soon collects, and divisions 

 occur, resulting in the formation of the early endosperm nuclei, 

 which lie free in the cytoplasm in the middle of the embryo-sac. 

 This cytoplasm, with its nuclei, later on, lines the wall of the sac. 



The fusion of the second generative nucleus with the definitive 

 nucleus completes the process known as double fertilisation, a 

 phenomenon which has recently been shown to occur in the 

 majority, if not all, of the Angiosperms. 



The fertilised egg-cell, or oospore, divides, after a short period 

 of quiesence, into two cells, viz. — an upper or epibasal cell, and 

 a lower or hypobasal cell, which bear an important relation to 

 the position of the rudimentary tissues to be shortly formed from 



