DIV U 



SPERMATOPHYTA 



573 



division. There may then be an egg-cell, an antipodal cell, and two polar nuclei, 

 or alternatively in Cypripedium an egg-cell, two synergidae, and one polar nucleus. 

 In all these cases the reduction division takes place in the embryo-sac, being trans- 

 ferred from the end of the sporophyte generation to the commencement of the 

 gametophyte generation. 



The microspores, which cannot reach the macrospore directly, 

 germinate on the stigma (Fig. 551). The pollen-tube penetrates for 

 the length of the style, and as a rule the tip enters the micropyle 

 of an ovule and so reaches the apex of the nucellus. This most usual 

 course of the pollen-tube is termed POROGAMY, but many cases of 

 departure from it have become known of recent years. 



FIG. 553. Ovule of Ulmus ])eduncuh/tu. 

 es, Embryo-sac; m, micropyle; ch, 

 chalaza ; t, pocket-like space between 

 the integuments. The pollen-tube, 

 ps, penetrates directly through the 

 two integuments and reaches the apex 

 of the nucellus. (After NAVVASCHIN.) 



FIG. 554. Longitudinal section of an ovary of 

 Juglans regia to show the chalazogamy. ps, 

 Pollen-tube ; e, embryo-sac ; cha, chalaza. 

 (Somewhat diagrammatic, x 6.) 



TREUB first showed in Oasuarina that the pollen-tube entered the 

 ovule by way of the chalaza, and thus reached the peculiar sporo- 

 genous tissue, which in this case develops a number of macrospores 

 or embryo-sacs. CHALAZOGAMY, as this mode of fertilisation is 

 termed in contrast to POROGAMY, has been since shown, especially by 

 NAWASCHIN, to occur in a large number of forms. These belong to 

 the Casuarinaceae, Juglandaceae, Betulaceae, Ulmaceae, Celtoideae, 

 Urtioaceae, Cannabinaceae, and Euphorbiaceae, which all have the 

 common character of the pollen-tube growing within the tissues, and 

 avoiding entrance by the micropyle. This in some cases (Urticaceae) 

 becomes closed or, as in the Euphorbiaceae, is covered by the obturator. 

 The pollen-tube makes its way to the embryo-sac sometimes from the 

 chalazal end and sometimes from the side of the ovule (Fig. 553), 



