272 



BOTANY OF THE LIVING PLANT 



The act of Fertilisation in the Higher Flowering Plants is then a 

 double one, involving the ovum and the central fusion-nucleus on 

 the one hand, and the two male gametes on the other. The nuclei 

 of the male gamete and ovum are both haploid, being cells of the 

 gametophyte generation, derived from spore-mother-cells which have 

 undergone reduction. The fusion of the gametes restores the original 

 number of chromosomes. The nucleus of the zygote is diploid, and that 

 diploid cell originates the new germ. The central fusion-nucleus had 

 already resulted from the fusion of the two polar nuclei. On its 

 fertilisation by the second male gamete a third nucleus coalesces with 

 it. This triple fusion is unique, so far as present observation extends. 

 It may have its physiological importance in relation to the develop- 

 ments that follow, for the triple fusion initiates the endosperm. ' 



FIG. 216. 



Fusion of the second male nucleus with the polar nuclei in Lilium Martagon. 

 A, an S-shaped male nucleus applied to the upper polar nucleus. B, the second 

 male nucleus (shown only in part) and the two polar nuclei close together. C, all 

 three nuclei fusing. (After Mottier.) 



An important feature characterising the intricate changes in the 

 embryo-sac of Flowering Plants is the extraordinary constancy in 

 the number and behaviour of the cells involved. Plants which differ 

 widely in form, internal structure and biological character, as well as 

 in the number and relation of their floral organs show a remarkable 

 uniformity in these details. Exceptions do exist, but they are few 

 relatively to the majority which conform. This indicates that 

 probably each step is significant in the success of the sexual propaga- 

 tion, though it is not possible to assign with certainty its exact function 

 to each. One general conclusion follows from comparison with forms 

 lower in the scale, though the foundations for it can only be given on 

 a later page ; it is that the parts directly involved in the sexual 

 propagation of Flowering Plants represent, in a vestigial form, parts 

 which are more fully represented in more primitive organisms. They 

 all belong to the Gametophyte, or Haploid Generation. Here they 

 appear as only a few cells contained on the one hand in the micro- 

 spore, or pollen-grain, and its tube ; on the other in the megaspore, 



