THE ANGIOSPERMAE 1453 



Pi= upper polar nucleus; P^,^ lower polar nucleus; Sp.= sperm nucleus. 



(P^-f P.^)-f Sp. This is the most frequent method, the polar nuclei uniting 

 before union with the sperm nucleus. 



(Pj-fPg + Sp.) Observed in Zea, Nicotiana, Fn'tillaria, Tiilipa and Gagea. 



(Pi + Sp.)+Po Observed in Monotropa. 



(P2 + Sp.)-fPi Observed in Adonis, where the antipodal polar nucleus 

 moves up to the egg-apparatus and unites with the sperm 

 nucleus, then retreats. Both the two last procedures occur in 

 Liliiun and in Nicotiana. 



(Pj-|-Sp.) This happens in embryo sacs where there is no antipodal 



polar nucleus, e.g., Oenotheraceae, Helosis and Limnocharis, 

 and has also been noticed in Adoxa, and perhaps in Lemna, 

 where direct fusion of the polar nuclei does not seem to 

 occur, but they divide co-ordinately and fuse during this 

 division. 



Division of the endosperm nucleus usually precedes that of the oosphere 

 and in many cases quite extensive formation of endosperm may take place 

 before the first division of the oosphere. Every degree of precedence may be 

 found in different plants, but it is rare for the oosphere to divide first, nor 

 is simultaneous division at all common. 



Since the days of Strasburger there have been two main types of 

 endosperm development recognized, the nuclear type and the cellular 

 type. A third type, of more restricted 

 occurrence, is the helobial type, 

 which is widespread among members 

 of the order Helobiales and is found 

 also in isolated cases in various 

 families outside this order. One or 

 two other anomalous types have been 

 described, the distribution of which is 

 not certainly known. 



In the nuclear type, the first divi- 

 sions of the endosperm nuclei are not 

 accompanied by cell wall formation 

 (Fig. 1333). The nuclei produced are 

 free in the cytoplasm of the embryo 

 sac and they may either remain free 

 indefinitely or else wall formation oc- 

 curslater, either progressively ormoreor 

 less synchronously throughout the sac. 



The first few divisions are generally synchronous, as in most multi- 

 nuclear cells, but this later gives way to irregularly distributed divisions, or 

 to waves of division spreading from one point throughout the sac. Whether 

 any cell plates are formed between the dividing nuclei is not yet clear, 



Fig. 



1333. — Uhmis americana. Free- 

 nuclear endosperm formation. A 

 shows an antipodal oosphere. B 

 shows an antipodal enbryo de- 

 veloping. {After Shattiick.) 



