CHAPTER XII. POLLINATION AND FERTILIZATION". 



/ 



Fig. 297. — Upper part of em- 

 bryo-sac of Torenia Asiatica, 

 after Strasburger. s, s, the two 

 synergidje capped by the filiform 

 apparatus/; o, the oosphere. 



end, and a nucleus located well toward the upper end ; the 

 oosphere, on the other hand, has its vacuole above and its 

 nucleus below. It usually also occupies a somewhat lower 

 position in the embryo-sac (see Fig. 

 296, o). At the lower or opposite end of 

 the sac are formed three cells, called 

 the antipodal cells, while somewhere 

 in the clear space between the latter 

 and the egg-apparatus, a nucleus may 

 be discerned. In many cases there is 

 formed on the upper part of the syner- 

 gidae a kind of spongy cap, consisting 

 of cellulose, which is traversed by 

 delicate pore-channels containing al- 

 buminous matter. This cap fills a per- 

 foration in the wall of the embryo- 

 sac, forming a kind of stopper to it ; 

 it is technically called the filiform ap- 

 paratus (see Fig. 297, f). 



The pollen-tube is guided to the micropyle, partly by various 

 mechanical contrivances, such as by papillae on the placenta, by 

 the position of the funiculi, etc., and partly, also, as it nears its 

 destination, by the stimulating influence of a fluid which escapes 

 from the synergidae through the filiform apparatus. Sometimes 

 the embryo-sac is penetrated by the pollen-tube — the ovules of 

 Canna afford an illustration of this kind ; but more commonly 

 it merely comes into contact with its surface, or with that of the 

 filiform apparatus. A transference of the fertilizing substance 

 then takes place, which is evidenced by important changes in 

 one, or sometimes both, of the synergidae. Their contents be- 

 come turbid, and then disintegration begins. These changes 

 are followed by remarkable ones in the oosphere. It becomes 

 invested in a delicate cell-wall ; a second nucleus, smaller than 

 the first, makes its appearance in the interior, and this, after a 

 little, unites with the one originally present. Soon after this, 

 cell-division takes place, and the oospore, as it is now called, 

 begins to develop into an embryo. Some of the successive 

 stages in this development are roughly illustrated in the dia- 

 grams, Figs. 298 to 301, inclusive. At first, cell-division takes 



