FERTILIZATION. 



416 



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 

 aiBnity. 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.— FertllUnUoii, 



1 longltniltoal seotlon through the ovule of Sphedm (a Qymnosperm) ; x 3. ' Apical portion of a longitudinal section of the 

 ovule ol SpM'Hm, sliowing the pollen-grains In the mloropyle producing their poUen-tuhes; xlOO. ' Longitudinal section 

 tlirough the ovai'y at Ornithogalum nutams; x 2. * Transverse section of the same ovary; x S. » Longitudinal section 

 through Btlgmo, style, and upper portion o( ovai'y of Omithogalum showing a p'oUen-grain on the stigma with its pollen- 

 tube passing down the style-oanal to tlie mloropyle of an ovule; x S. "Longitudinal seotlon through an ovule of 

 Umithogalum. The funlole 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-oell and the other a synerglda, at the base two antipodal cells are represented. Around the embryo-sao is a layer of 

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

 inlegnraents, A pollen-tube has grown down the mloropyle and perforated the apex of the embryo-sac. It is represented 

 In contact with the egg-oell and one synerglda. The other synerglda is not shown; x 100. (Pai'tly 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 



