THE ANGIOSPERMAE 1445 



the tube. Comparisons with the spiral form of many free antherozoids have 

 naturally been drawn, though whether the peculiarity of form has any con- 

 nection with motility is uncertain, as it is 

 not by any means universal. The move- 

 ment of the sperm cells down the tube 

 may be purely passive and attributable 

 to cytoplasmic streaming but it has been 

 pointed out that while the growth of the 

 tube is continuous, the distance of the 

 sperm cells from the end is variable, 

 which suggests autonomous movement, 

 perhaps by excretion pressure. They 

 show no active movements comparable 

 to those of antherozoids. 



The tube nucleus may persist (Fig. 

 1326) and even enter the embryo sac, but 

 it generally becomes amoeboid, may be 

 greatly and irregularly elongated and has 

 been described as fragmenting. It loses 

 its affinity for basic stains whereas the 

 sperm nuclei stain deeply. 



The Act of Fertilization 



When the pollen tube reaches the 

 embryo sac it penetrates its membrane. 

 The latter is often, perhaps always, 

 delicate and non-cellulosic at its upper 

 pole, no more than a plasmatic mem- 

 brane or a sheath of pectin. Accounts p,^ i^zb.-Hyadnthus orientals. 

 of subsequent events differ and there Pollen tube from a culture, 



.1 . ^u„ . „_^ showing swollen tip of tube, and 



IS no reason to suppose that they are ^j^^ elongate tube nucleus fol- 



necessarily of uniform character. Well- lowed by the two generative 



supported observations show that the ""'^ ^^' 



tube may enter one of the synergid cells and there discharge its nuclei. 

 This is followed by enlargement of the basal vacuole in the synergid cell, 

 which then bursts, discharging its contents, including the pollen tube nuclei, 

 into the embryo sac " with some vehemence " as Steffen says. The syner- 

 gid is thus destroyed. This looks like a rather specialized discharge mechan- 

 ism and it can only be set off once, since if a second pollen tube enters the 

 other synergid, no reaction follows. Naturally nothing of the kind can occur 

 where the synergids have not been formed or have already degenerated before 

 fertilization and in many plants the pollen tube has been seen passing between 

 existing synergids and penetrating more or less deeply into the embryo sac 

 before discharging. The mechanism of discharge is still obscure, but it has 

 been shown in pollen tube cultures that the end of the tube is very delicate 



