H50 



A TEXTBOOK OF THEORETICAL BOTANY 



discussed later under Apomixis. Here we may simply mention that 

 dispermic fertilization of the oosphere sometimes occurs, but Steffen esti- 

 mated it at only 0-3 per cent, of the examples he observed in Impatiens 

 glandidigera. 



Fig. 1332. — Impatiens i^hmdiili^era. Entry of secondary 

 male nucleus into the primary endosperm nucleus. 

 A and B, Phases of entry. C, Nuclear membrane of 

 male nucleus has disappeared and the chromatin is 

 dispersing. B shows beginning of chromatin re- 

 generation in the endosperm nucleus. (After Steffen.) 



The uniting nuclei are generally said to be in the resting stage or else to 

 show the appearance of early divisional prophase. If the essential feature 

 of bi-parental inheritance is held to be the pairing of two sets of homologous 

 chromosomes, then nuclear union cannot be considered complete until this 

 has taken place, which cannot be until the first zygote division. Observations 

 differ on whether two groups of chromosomes are still distinguishable in the 

 early stages of this division, but it is at metaphase, at latest, that their inter- 

 mingling must be completed. 



In the primary endosperm nucleus separation of the chromosomes is 

 maintained longer than in the fertilized oosphere. Distinct groups of 

 chromosomes may still be evident at the second division, but it is not 

 known how long this state persists. Tripolar spindles may be formed at 

 the second division which are reduced to bipolar by the drawing in of the 

 third arm. 



The interesting question has often been raised, whether fertilization in 

 the Angiosperms is entirely indiscriminate or whether such a thing as 

 selective fertilization exists, either by a truly selective attraction between 

 certain pollen tubes and certain ovules or else by competition. The latter 

 must certainly be an element in some cases, where pollen tubes exhibit 

 differential rates of growth. The genie composition of the pollen may control 

 the rate of growth, as Buchholz has already shown in Datura. Some genes 

 are lethal or semi-lethal to pollen tube growth and seem to affect the game- 



