1420 A TEXTBOOK OF THEORETICAL BOTANY 



Obviously what has just been said apphes only to the normal condition 

 where a pair of polar nuclei are formed, though this is true of the vast 

 majority of cases. In four-nucleate sacs of the Oenothera type there is only 

 one polar nucleus, which behaves as if it were the primary endosperm nu- 

 cleus. In sixteen-nucleate sacs and even in some rare instances of eight- 

 nucleate sacs {Plumbago) the number of nuclei combining to form the primary 

 endosperm nucleus may be from three to fourteen, with a correspondingly 

 high degree of polyploidy in the endosperm nucleus. In Fritillaria and 

 other genera of its type {e.g., Liliiim) the lower polar nucleus, like the anti- 

 podals, is triploid and the primary endosperm nucleus before fertilization 

 is thus tetraploid. Among the Balanophoraceae there are some types where 

 the upper polar nucleus functions alone as the primary endosperm nucleus, 

 while the lower polar nucleus combines with the three antipodals to form an 

 evanescent structure. 



If the two polar nuclei meet in the centre of the sac, then they subse- 

 quently move, or their fusion product moves, up to a position close beneath 

 the oosphere, if not in actual contact with it. Beneath them there develops 

 a single very large vacuole. When they remain in a central position, or if, 

 as occasionally happens, they move downwards to the antipodal end, there 

 is a thick cytoplasmic strand linking them to the oosphere. The antipodal | 

 position is quite uncommon and is always associated with the formation of 

 cellular endosperm, where the first division wall traverses the embryo sac 

 at its lower end. 



The cytoplasm of the big cell, the endosperm primordium, frequently 

 contains starch in notable amounts. This food reserve may have a double 

 value, either in ensuring the continued life of the embryo sac under condi- 

 tions where there is considerable delay in fertilization, from whatever 

 natural cause this may arise, or alternatively in enabling the rapid develop- 

 ment of the endosperm after fertilization. The starch reserve is used up in 

 one way or the other and disappears after fertilization. It is not continuous 

 with the reserves which appear in the endosperm cells at a later stage. 



The Antipodals. Normally there are three antipodal cells, but as will 

 have been seen from the descriptions already given of the various types of 

 embryo sac (p. 1406) there are many exceptions; indeed, this is the most 

 variable part of the embryo sac. Great differences exist in the size, the 

 number, the nuclear condition, the durability and the fate of the anti- 

 podals. In tetranucleate sacs of the Oenothera type they are altogether absent. 

 This is also the case in Podostemaceae and in individual cases in other 

 families, e.g., Alchemilla. Many other plants show, however, so rapid a 

 degeneration and disappearance of the antipodals that the mature embryo 

 sac may have none, even though they have previously been formed. Ex- 

 amples of such evanescent antipodals, which are usually small and insignifi- 

 cant, may be found in the following families, among others: Salicaceae, 

 Potamogetonaceae, Caryophyllaceae, Hypericaceae, Aceraceae, Verbenaceae, 

 Campanulaceae, Juncaceae. The condition is thus widespread and not 

 limited to any particular class of Angiosperms. 



