98 Oliver . — On the Structure , Development , 
end of the embryo-sac elongates, penetrating the tissues, but 
neither is there any homology here. 
It is interesting to remember that amongst the few plants 
in which it is stated no endosperm is at any time formed, 
Hofmeister gives Trapa as one (also Zoster a, Naias , Tro - 
paeolum, and Grchidaceae). 
Anatomy of Vegetative Organs. 
The Stem. — In Trapella the stem has an average thickness 
of i*5 mm., and conforms in its general structure to the 
normal, reduced, aquatic type. It consists of an axial vascular 
cylinder, surrounded by a wide parenchymatous cortex freely 
supplied with air-spaces. In different regions of the stem 
these spaces differ in their extent. No peripheral bundles 
whatever are present, and the vascular cylinder — which runs 
unbranched throughout the internodes — is delimited by a very 
well-marked endodermis (ens., Figs. 49-51). In the adult 
parts of the stem, the radial walls of the endodermis are 
thickened in a very characteristic manner (Fig. 51). 
In the axial bundle-cylinder, and immediately surrounding 
a central c pith,’ is a ring of vessels, constituting the xylem 
(vv., Fig. 51), which, in its distribution, recalls that of Hippuris 
vulgaris. Outside this is a zone of thin-walled tissue, which 
is the phloem, and consists, for the most part, of sieve-tubes. 
Externally this zone is limited by a well-marked endodermis 
(ens., Fig. 51). The ‘ pith 5 consists of thin-walled parenchyma- 
cells, oblong in longitudinal section, and with longitudinal axes 
about three times as great as the transverse. They contain a 
number of small starch-granules. Here and there, in the pith, 
an intercellular space (i.s., Fig. 51) is seen, due to the breaking 
down of spiral vessels ; in them, at places, the remains of the 
spiral coil may be seen remaining. The vessels are both spiral 
and scalariform ; in the former case the spire is a close one. 
The cells which immediately abut on the outside of the ring 
of vessels are wide and resemble those of the pith. 
Next follows a zone of sieve-tubes, with a width varying 
from two to three tubes. Sieve-plates occur only upon the 
