574 BOTANY TART n 



penetrating the tissues that lie between it and the egg apparatus. 

 Since, according to the opinion of many authors, the families 

 mentioned above stand at the lower end of the series of Dicotyledons 

 where a connection with the Gymnosperms might be looked for, this 

 type of fertilisation may be regarded as departing from the behaviour 

 of the more numerous porogamic Angiosperms and approximating to 

 the original relations in Gymnosperms. In the latter the whole over- 

 lying tissue of the nucellus has to be penetrated by the pollen-tube 

 to reach the embryo-sac (Figs. 545, 553, 554). 



In a more recent work NAWASCHIN shows that there are also 

 indications in the development of the contents of the pollen- tube that 

 these forms are at a lower stage than the majority of Angiosperms. 



In Juglans the two generative nuclei remain 

 enclosed by a common protoplasmic mass 

 K (^g- 555) which even enters the embryo- 



sac > ^ then gradually disappears and the 

 naked nuclei emerge and fulfil their re- 

 spective functions. NAWASCHIN points 

 out that these relations agree with what 

 is found in many Gymnosperms. Thus 

 WM cL they afford a further clear indication that 

 the transition from the latter group is 

 to be looked for in these lower families 

 of the Angiosperms. 



When the pollen-tube, containing the 

 two generative cells, has reached the 



Fir, ^.-Juglans nigra. a, Part of ern b r y O -saC, its Contents CSCape and pass 

 the embryo -sac in longitudinal . J r . 1 



section before fertilisation, sho*- by way of one of the synergidae to the 

 ing the relation of the bimicieate ovum ; the corresponding synergida then 



rCS:S dies - One of the two s enerative 



magnified. (After NAWASCHIN.) f US6S With the IlUCleUS of the OVlim, w 



then becomes surrounded by a cellulose 



wall. The second generative nucleus passes the ovum and unites with 

 the large secondary nucleus of the embryo-sac to form the ENDOSPERM 

 NUCLEUS (Figs. 556, 557). Both the male nuclei are often spirally 

 curved like a corkscrew, and NAWASCHIN, who first demonstrated the 

 behaviour of the second generative nucleus, compares them to the 

 spermatozoids of the Pteridophyta. The further development usually 

 commences by the division of the endosperm nucleus, from which a 

 large number of nuclei lying in the protoplasm lining the wall of the 

 embryo-sac are derived. The endosperm arises by the formation of 

 cell walls around these nuclei and their proper surrounding proto- 

 plasm, and by the increase in number of the cells thus formed (Fig. 

 565 A) to produce a massive tissue. 



The distinctive feature of the development of the endosperm in 

 Angiosperms from the prothallus of Gymnosperms -lies in the 



