MR. J. MIERS ON THE FAMILY OF TRIURIACER. 55 
be compared with the arilliform covering above described in the genus just mentioned. 
The embryo of Pistia is cylindrical, nearly half the length of the albumen, in the summit 
and in a longitudinal cavity of which it is placed, with its apex quite bare, the whole being 
enveloped by a thin pellicular integument, and this again by a thin and somewhat coria- 
ceous reddish testa, marked much after the manner of that of Soridium. This testa is 
fixed upon a long, thickened, stipitate support, and is crowned at its apex by a dark 
pulvinate process, suspended by a cylindrical plug or strophiole, the point of which is seen 
in the umbilical apex of the seed. The external covering is a thick, wrinkled, fungous or 
pithy substance, lined inside and outside by a thin adhering membrane, and enclosing the 
stipes, the testa and the strophiole, and it can be considered in no other light than an arillus, 
and as analogous to the peculiar covering of the testa seen in Sciaphila. In the Zingibera- 
cec the seed is also covered by an arillus somewhat similar to that of Pistia, while in Rave- 
nala, by its numerous ribs, it approaches yet nearer to the seminal envelope of Sciaphila; 
here, however, as well as in Pistiee, the albumen is of peculiar structure, being farinaceous 
when dry but becoming distinctly cellular when moistened, and the cells are separable 
from each other without bursting; they are then translucent, and bear altogether a very 
different aspect from those seen in the nucleus of Sciaphila, in which they are more 
opake, as if filled with dark grumous matter. The facts here brought together are inter- 
esting, as presenting some analogies bearing upon the question, and they serve to show 
that we have yet much to learn concerning the nature of the more simple forms of em- 
bryonal structure, and of the functions of reproduction. 
In respect to the position of the Triuriacee in the system, if we follow the rules of 
elassification founded upon the three great divisions of Acotyledonous, Monocotyledonous, 
and Dicotyledonous plants, a plan now quite untenable, they must of course range in the 
same group as the Balanophoree, although they hold but little relationship with them. But 
if we regard the condition of the embryo, not as a basis, but merely as a frequent indication 
of the three great divisions, founded on the structure and development of vegetable fibre, 
viz. Acrogens, Endogens and Exogens, and if we accord with the views of Mr. Brown and 
Mr. Griffith, in considering what the latter calls the homogeneous-embryo-form state of 
the seed merely as an imperfect condition of development, common alike to all these 
groups, then the Triuriacee must take their place among the Endogens. 
Here, the family that at first sight appears most approximate is the Alismacee, with 
which the Triuriaceæ agree, in their simple stem, sometimes spicate unisexual flowers, 
and their numerous carpels; but Alismacee differ in having leaves with parallel HERVOR, 
in their floral envelope being distinctly biserial, tlie outer calyeine, the inner petaloid, and 
both with imbricated æstivation ; they are also dissimilar in their bilocular anthers, with 
parallel cells, dorsally affixed to long filaments ; ovules often two in each carpel, Sue being 
superimposed ; carpidia opening by their ventral suture, and seeds with large hippocrepi- 
form embryo. : ^ 
With Fluviales, Triuriacee accord in their soft cellular structure, their moncecious 
flowers, simple perianthium with valvate æstivation, often 4-locular anthers, several 
distinct ovaria, with a single erect ovule in Caulinia and Najas, baccate fruit, with a 
pericarpial utricle bursting on one side in Althenia, Zostera, Najas and Cymodocea; but 
