274 Cook: EMBRYOLOGY OF RHIZOPHORA 
ing obliquely from the surface of the embryo outward and upward 
towards the integument (sac), indicating that the growth of the 
embryo has exerted great pressure on them (FIGURE 12). Up to 
this time there has been very little modification in the surface cells 
of the cotyledons; they are more protoplasmic than the inner cells 
but otherwise not materially different (FIGURE 26). The growth 
of the cotyledons now almost entirely ceases and the second period 
of growth, or growth of the hypocotyl, begins. The hypocotyl 
grows very rapidly, pierces the apex of the ovary and forms the 
long pendulous structure, which is frequently eight or ten inches 
in length (FiGuREs 14, 17). The plumule (FIGURE 15) and the 
vascular tissues of the cotyledonary structure which originated 
considerably earlier (FIGURE 10, v.s.) become more prominent and 
important. The entire cotyledonary structure undergoes consider- 
able change; the cells appear to contain more protoplasm and 
stain more deeply. The surface cells and those near the surface 
become very much elongated with their long axis at right angles 
to the surface (FIGURE 27). All these modifications are probably 
for the increased activity of these cotyledonary structures, which 
undoubtedly absorb great quantities of food for the development 
of the long hypocotyl. At about this same time numerous 
masses or strands of very much elongated cells are formed, which 
lead from the surface cells to the vascular system of the cotyle- 
dons (FIGURE 16). A more careful examination of these strands 
of cells shows them to be of the same character as the vascular 
system (FIGURES 29, 30), and the author suspected that this sys- 
tem contained more or less sieve tissue but was unable to decide 
positively on that point. Just before the separation of the hypo- 
cotyl from the cotyledonary structure, traces of tracheary tissue 
(FIGURES 29, 30) appear but it is never very abundant. The endo- 
sperm over the surface of the cotyledonary structure becomes 
very thin with the corresponding slow growth of these organs 
during this period, and the cells of the integument (wall of sac) 
are very rich in protoplasm (FIGURE 28). 
When the hypocotyl is about three inches in length, about two- 
thirds of the cotyledons are imbedded in the embryo-sac, and the 
remaining part in the ovary-chamber, surrounded by a thin layer 
of endosperm, while the hypocotyl projects through the apex of 
