certain Penaeaceae . 
369 
mosomes, and usually several nucleoli (cf. Figs. 31-3). The endosperm 
exists at first only as a number of nuclei, scattered through the parietal 
layer of cytoplasm which surrounds the large central vacuole that is formed 
as a result of the growth of the embryo-sac (cf. Figs. 27, 28 a, 30 a). As 
may be seen in Figs. 28, 31-3, at the micropylar end of the sac, this layer 
closely invests the embryo. In the thicker protoplasm found at the lower 
end of the sac as it extends downwards into the nucellus, from three to eight 
of the endosperm nuclei become associated in a deeply-staining group, their 
nucleoli at the same time fusing to form one large and deep-staining nucleolus 
in each. This group is doubtless haustorial in function, passing on to the 
rest of the embryo-sac food obtained from the disorganization of the axial 
cells of the nucellus. After these cells have been absorbed the group is no 
longer recognizable. It is not until the embryo has attained to the size shown 
in Fig. 33 that any cell-walls are formed in the endosperm. Cell-wall- 
formation continues until by the time the cotyledons have appeared (Fig. 35), 
the greater part of the embryo-sac is filled with a delicate endosperm tissue. 
This is all used up by the developing embryo, which grows down into it. 
Embryo. 
The most striking fact in connexion with the development of the 
embryo is the absence of any form of suspensor in even the youngest stages. 
The first wall formed is transverse (Fig. 27) ; a longitudinal wall is then 
formed in each cell (Figs. 28, 29 b), and the whole of the spherical proembryo 
thus formed enters into the formation of the embryo, which preserves this 
spherical outline until the formation of the plerome (cf. Fig. 32). The 
dermatogen early becomes differentiated (Figs. 30, 31), and the plerome, 
is marked out at a slightly later stage (Figs. 32, 33). The embryo begins 
to elongate (Figs. 33, 34), and the cotyledons are formed (Fig. 35). They 
are very slightly developed (Fig. 36), the storage function, which is their 
normal one in an exalbuminous seed, being here performed by the unusually 
massive hypocotyl, whose cells are closely packed with starch. This great 
development of the hypocotyl, and its adaptation thus early in life to the 
function of food storage, may very probably be considered as an adaptation 
to a xerophilous habit. The seedlings of geophilous xerophytes have been 
shown to exhibit two marked characteristics 1 — a tendency towards reduction 
in the size of the cotyledons (usually by a partial fusion, or by the abortion of 
one), and an adaptation of the hypocotyl for the storage of food after the seed 
has germinated. In this strongly xerophilous and probably ancient order it is 
then not surprising to find similar characteristics so marked even in the seed. 
The root-tip of the embryo in the mature seed (Fig. 37) has some 
peculiar features. It is of unusual breadth, and the growing point is 
situated at the bottom of a slight depression. There is no root-cap, and the 
Sargent, 1903, pp. 78-81. 
