On Some Stages in the Life History of Gnetum. 243' 
cells, become a permanent part of the endosperm. In other words, the 
endosperm of the micropylar region is formed in exactly the same manner 
as that of the chalazal end. It is, therefore, a little misleading to call the 
micropylar region " fertile " and the chalazal " sterile." * Both regions 
alike produce sterile (endosperm) tissue and in the same manner ; fertilisa- 
tion is usually, though possibly not always, confined to the micropylar region 
merely because the pollen-tube enters it and the sperm-nuclei there meet 
with free female gametes. 
Owing, no doubt, to the disturbances caused by the entrance of the 
pollen-tube and the formation and growth of the oospores, it frequently 
happens that a few of the micropylar sac-nuclei, usually situated close to the 
wall of the sac, escape enclosure in an endosperm compartment (Plate 
XLIX, figs. 25 A, b). These do not fuse, and their appearance and reactions 
suggest that they play no further part in the economy of the sac ; they 
occupy a position in the life-history which corresponds to that of the gametes 
in the embryo sac-tubes of Welwitschia which remain unfertilised. The 
" crowding out " of a few nuclei in this manner is not uncommon ; a similar 
case is recorded by Lotsy.f The fact that this occurs at all is evidence for 
the view expressed later (p. 262) that the whole process of septation and 
nuclear fusion is due, in the main, if not entirely, to causes which are 
inherent in the physical and physiological conditions of the sac. Any of the 
sac-nuclei may be fertilised ; any or all of them may fuse in groups after 
septation, producing endosperm ; if, owing probably to simple mechanical 
causes, a number of them are excluded from both possibilities, neither the 
results of fertilisation nor of the nuclear fusion which produces primary 
endosperm cells are in any way interfered with, and the excluded nuclei have 
no future. These facts involve physiological questions of deep significance, 
but from a morphological point of view we are mainly concerned with the 
phylogeny of the nuclei ; the fusion inter se is probably a new character, 
whose origin is to be sought in the conditions under which they are formed. J 
The endosperm later increases greatly in amount and becomes to some 
extent differentiated. Two massive growing points are organised, one at 
each end ; that at the chalazal end is more active than the other ; a result of 
its activity is the destruction of the pavement tissue § of the nucellus. The 
micropylar end advances into the nucellar cap ; in the most advanced stages 
seen by us, this region of the nucellus is reduced to a thin plate of crushed 
and disorganised cells (Plate XLVIII, fig. 22 a, N). The peculiar form of 
the upper end of the advanced endosperm described by Coulter || for 
Gnemon has been seen in this species but not in G. africaiium. 
* Lotsy, loc. cit. 
t Lotsy, 1899, fig. 44, W. o. E. 
X Gf. Coulter, 1911 ; Pearson, 1915 a, p. 329. 
§ Coulter, 1908, figs. 2-3 A; Pearson, 1915 a, fig. 19. 
I] Coulter, loc. cit., p. 45 ; fig. 6. (0/. Welwitschia, Pearson, 1909, figs. 89 I.E.) 
