FERTILIZATION. 



415 



considerable distances down the style, though they perish eventually. Still even 

 should these foreign pollen-tubes actually get access to the micropyle, a fertilization 

 of the egg-cell by the foreign spermatoplasm would be impossible, owing to the 

 inability of the sexual protoplasm to combine outside a very narrow range of 

 affinity. We have already seen that many plants can be fertilized either by pollen 

 from another flower of the same species, or, wanting this, by their own pollen. 



Fig. 316.— Fertilization. 



' Longitudinal section through the ovule of Ephedra (a Gymnosperm) ; x 3. 2 Apical portion of a longitudinal section of the 

 ovule of Ephedra, showing the pollen-grains in the micropyle producing their pollen-tubes; xlOO. 3 Longitudinal section 

 through the ovary of Ornithogalum yiutans; x 2. * Transverse section of tlie same ovary; X 3 5 Longitudinal section 

 through stigma, style, and upper portion of ovary of Ornithogalum showing a pollen-grain on the stigma witli its pollen- 

 tube passing down the style-canal to the micropyle of an ovule; x 3. ^ Longitudinal section through an ovule of 

 Ornithogalum. The funicle or stalk of the ovule is seen to the left, the ovule proper to the right. In the latter there is a 

 large central space, the embryo-sac which contains certain small cells; towards the apex two together of which one is the 

 egg-cell and the other a synergida. at the base two antipodal cells are represented. Around the embryo-sac is a layer of 

 nucellar tissue one cell thick, whilst below, this tissue is more bulky (contents dotted). Around the nucellus are the 

 integuments. A polleu-tube has grown down the micropyle and perforated the apex of the embryo-sac. It is represented 

 in contact with tlie egg-cell and one synergida. The other synergida is not shown; xlOO. (Partly after Strasburger.) 



Both categories of pollen-grains are competent to develop tubes and to fertilize the 

 ovules. Under these circumstances it would be very interesting to know what 

 exactly happens when pollen-grains of both these categories are present on one and 

 the same stigma; whether (as is probable) both develop pollen-tubes, whether both 



