THE SALT-MARSH AGAIN 203 
Here the absence of higher plants from the areas 
below high water, as compared with their abundance 
above water-level, is a conspicuous feature. This is a 
noteworthy point, because if we assume that the 
presence of salt is the main factor which has pre- 
vented the land plants from spreading downwards, we 
are faced with the fact that the soil of the salt-marsh, 
where many such plants occur, may by evaporation of 
water become much more highly charged with salt 
than the sea itself. Yet the salt-marsh flora includes 
representatives of many Natural Orders, including 
some of the most highly specialized families— 
Ranunculacee (R. sceleratus), Crucifere (Cochleana 
spp.), Caryophyllacee (Alsine), Umbellifere (Apium 
graveolens, Genanthe Lachenalu), Composite (Aster 
Tripolium, Artemisia maritima), Primulacee (Glaux 
maritima), Plumbaginee (Statice, Limonium). It 
seems clear that it is the assumption of the marine 
habit which is the stumbling-block, not the presence 
of salt. The Grass-wrack or Zostera, our only marine 
Seed Plant, comes of one of the oldest stocks of 
aquatic plants, and its nearest relatives have long been 
toying with the idea ofa maritime habitat. The Order 
to which it belongs, the Naiadacee or Pondweed 
family, from their worldwide range, their number, 
their variety, and their uniformly aquatic habit, may 
be set down as among the earliest Seed Piant colonists 
of lakes and rivers; some of them favour brackish 
water, while others besides the Grass-wrack have 
taken to marine life. Without going beyond the 
limits of our native Naiadacee@ we can study the 
various stages, and form a picture of how the Grass- 
wrack migrated to the sea. First we have the 
