28 NAIADACE 
overlap by their base, and turn backwards at the point. The small roundish 
green spike appears in July and August, and the plant is common locally in 
pools, ditches, and slow rivers, from the Forth of Clyde to Kent and Somer- 
set ; and in Ireland. 
** * * Leaves all submersed, stipules sheathing. 
21. Fennel-leaved Pond-weed (P. pectindtus).—Leaves bristle-shaped, 
1—3-ribbed, parallel, arranged in two rows; spikes interrupted. This plant 
varies much in the length and size of its stems and leaves, but is distinguished 
by its tuberous rootstock. It could not be confounded with any other species 
but the following, from which it differs chiefly in having its spikes less inter- 
rupted, and its fruit being smaller. The nut is also ribbed on the back, 
while that of the next is without ribs. It is of a bright green colour, 
resembling fennel, when lying in masses in the waters, and its stem is much 
branched. The spikes appear in July, are few in number, and slightly inter- 
rupted. It is a rare plant, inhabiting ponds, streams, and salt marshes. A 
form of this is recognised as a sub-species under the name of P. flabellatus. 
22. Slender-leaved Pond-weed (P. /iliférmis).—Stem thread-lke ; 
leaves hair-like, 1-nerved, sheathing by their stipules. This plant has spikes 
which are greatly interrupted, and grow on long foot-stalks. It flowers in 
June and July. It occurs in lakes and ditches at Anglesea, locally from 
Berwick northward ; and in Ireland rarely. 
2. Ruppia (Levippia). 
Sea Ruppia, or Tassel-grass (2. maritima).—Stems long, slender, 
round, much-branched, leafy ; leaves linear, opposite and alternate ; perennial. 
This is a plant which, growing in salt-water pools and ditches, might at first 
be taken for the Fennel-leaved Pond-weed, P. pectinatus. It has slender, 
much-branched stems, scarcely thicker than a packthread, but a couple of feet 
long, and long slender bristly leaves with sheaths, which are often very 
conspicuous. But in July and August one may see plainly that this plant is 
not a pond-weed, but that it has a very peculiar structure. The greenish 
flowers of this Ruppia grow in pairs, on a spike, which is enclosed at first in 
a membranous sheath, and they have neither calyx nor corolla, but their 
two anthers are very large. Aquatic plants have mostly some means of 
raising themselves above the water during the time of flowering, and this 
process is effected in the Tassel-grass by means of the flower-stalk, which 
lengthens at this season to some inches, according to the depth of the water, 
and coils itself up in a spiral form, so as to reach the surface. The Rey. 
Gerard Edward Smith remarks upon this plant, that he observed the pollen 
scattered over the water, and the small yellow spikes rising above its level, 
many having already discharged their pollen, while in a few the elevated 
anthers were yet entire. He adds, ‘The anthers are vesicular and buoyant; 
as they swell and become mature, the membranous sheath enclosing them is 
distended, and the whole is brought to the surface of the water. The flower- 
stalks are rapidly lengthened, the flowers quit the sheath, which then becomes 
a bladder, and aids the elevation of the spike an inch above the water. 
Presently the anthers burst, the vesicle loses its buoyancy, and the flower- 
