238 INTRODUCTION TO CYTOLOGY 



7 hours; Phajus grandifolius, 2 months; Hicoria pecan, 5 to 7 weeks; 

 Datura stramonium, 25 hours at 20°C.; CEnothera ruhrinervis, 36 hours; 

 Quercus rubra, 13 to 14 months. In Beta vulgaris, growing under summer 

 field conditions, the period is 20 hours, and a period of equal length 

 elapses before the fertilized egg divides (Artsch wager, unpubl.). It has 

 been shown in certain cases that the rate of pollen-tube growth varies 

 with the temperature and other conditions, which accounts in part for 

 the discrepant reports of different observers. 



The Endosperm Fusion. — The second male nucleus (it may be either 

 the first or the second to emerge from the pollen tube) passes to one or 

 both polar nuclei or to the product of their fusion. The fusion of the 

 three nuclei to form the primary endosperm nucleus is carried out in a 

 variety of.ways. The most commonly reported mode is that in which the 

 two polars unite to form a polar-fusion nucleus ("embryo-sac nucleus") 

 before the entrance of the pollen tube, the male nucleus being added later. 

 Less frequently the male nucleus meets and fuses with the polar nucleus of 

 the micropylar end of the sac, the other polar then fusing with the 

 product. This is the method described by S. Nawaschin (1898, 1899a) in 

 his accounts of the discovery of double fertilization in Liliuni Martagon 

 and Fritillaria tenella. In Zea the male nucleus and one polar complete 

 their fusion while the two polars are in contact but still unfused (V. 

 Rhoades, unpubl.). The simultaneous fusion of all three nuclei appears 

 to be a common occurrence ; it has been described in some detail by Noth- 

 nagel (1918) for Trillium and Lilium. Different modes may occur in 

 the same plant under varying conditions, notably of temperature (Shibata, 

 1902, on Monotropa). Although the three nuclei often appear exactly 

 alike, it is frequently possible to distinguish the male from the polars, 

 not only by its shape and smaller size, but by the condition of its karyotin. 

 In Lilium longiflorum (Weniger, 1918), for example, the male nucleus is in 

 the prophase while the polar nuclei are still reticulate. The membranes 

 of the three nuclei may persist for some time after they come into intimate 

 contact, and even after they have disappeared the chromatic elements of 

 the three constituent nuclei may in many cases be distinguished if the 

 section has been made in a favorable plane. When fusion occurs in the 

 metabolic stage this is not so apparent, but when it occurs in the prophase 

 the three chromatic groups are made out with little difficulty. 



As the division of the endosperm nucleus approaches, the chromatic 

 elements of the three contributing nuclei become increasingly distinct, 

 even if one or more of the nuclei have fused in the reticulate condition 

 (Fig. 144, F). As the prophase proceeds, all of the chromosomes (the 

 triploid number) are seen to be split longitudinally. Since this is repeated 

 in every subsequent mitosis, the resulting endosperm nuclei are normally 

 all triploid, each of them having one paternal and two maternal chromo- 

 some sets. In most angiosperms the developing endosperm passes 



