LAND PLANTS IN THE SEA 205 
sion to water-pollination is found. When the very 
simple male flowers shed their pollen, the grains, 
which are heavy owing to the presence of starch, fall 
through the water on to the female flowers which are 
borne below them, or are carried by currents to other 
flowers. Lastly we come to the Grass-wracks, a small 
group of submersed marine plants. While some of 
them, like our little native Z. nana, haunt muddy sands 
between tides, our more familiar species, the common 
Z. marina, is thoroughly marine, growing tall and 
vigorous among the large Seaweeds down to far 
below low-water mark (to over 30 feet in the Baltic). 
The plant has, nevertheless, not yet developed sub- 
mersed pollination, the pollen-grains rising to the 
surface, where they are caught by the stigmas of 
floating female flowers. It follows that the indi- 
viduals rooted in the deeper water, though growing 
vigorously, do not mature seed, for the production of 
which the species has to rely on plants which, at least 
at low water, are rooted sufficiently near the surface 
_to allow the flowers to rise above it. Could the 
species achieve submersed pollination, it appears quite 
capable of colonizing throughout the Laminarian 
zone, wherever there is a soft substratum for its 
creeping stems. 
The land plants of the salt-marsh, as well as the 
aquatic species, furnish interesting examples of over- 
lap with the sea flora, but a brief reference must suffice. 
The Glasswort (Salicornia), for instance, has furnished 
itself with a very complete equipment for the difficult 
conditions of salt-marsh life (see pp. 17, 18), and grows 
far out on the mud-flats in green colonies, often 
below the upper limit of the Bladder-wrack or Fucus, 
