Embryo in Lilaea subulata , //. B. K. 21 
of the members of the embryo to the young segments is quite 
evident. 
Owing to the formation of vertical walls in all of the lower 
segments, there is usually no secondary suspensor, such as 
usually occurs in other forms, but the enlarged vesicular 
primary suspensor-cell is in direct contact with the basal cells 
of the embryo (Fig. 43). 
The whole of the cotyledon, and possibly also the stem- 
apex, is derived from the terminal segment of the young 
embryo ; but owing to the late period at which the stem-apex 
is first recognizable, it is impossible to decide positively 
whether, as in Naias , it originates from the second segment, 
i. e. the cell immediately below the terminal quadrant-cells, 
or whether, as in Z annichellia^ it is the product of the terminal 
quadrants (Figs. 40, 41). The cell-divisions in the terminal 
region proceed with great rapidity, while in the basal segments 
growth proceeds much more slowly. The embryo in con- 
sequence becomes pear-shaped, the lower narrow portion 
being the product of the basal segments, while the enlarged 
upper part is derived entirely from the terminal segment. 
Fig. 39 shows a nearly median longitudinal section of an 
embryo where the primary tissue-systems are clearly defined, 
but the external differentiation is not yet indicated. The 
central plerome-strand is well marked, and shows in longi- 
tudinal section two rows of cells, separated from the dermatogen 
by the periblem, which is for the most part composed of two 
layers. 
In an older stage (Fig. 40) the plerome-cells have under- 
gone longitudinal divisions, and the periblem, especially in 
the cotyledon, has become very much more massive, owing 
to cell-divisions in all directions. Cross-sections of the young 
embryo are oval in outline (Fig. 44), and in the middle region 
{c) show a somewhat evident differentiation of the primary 
tissues, which is not clear in the basal region (a, b). 
The first evidence of external differentiation is a slight 
depression on one side of the embryo, near the base. This 
marks the position of the stem-apex : but it is difficult to 
