The Collaborative International Dictionary of English v.0.48:

Marsh \Marsh\, n. [OE. mersch, AS. mersc, fr. mere lake. See
   Mere pool, and cf. Marish, Morass.]
   A tract of soft wet land, commonly covered partially or
   wholly with water; a fen; a swamp; a morass. [Written also
   marish.]
   [1913 Webster]

   Marsh asphodel (Bot.), a plant (Nartheeium ossifragum)
      with linear equitant leaves, and a raceme of small white
      flowers; -- called also bog asphodel.

   Marsh cinquefoil (Bot.), a plant (Potentilla palustris)
      having purple flowers, and found growing in marshy places;
      marsh five-finger.

   Marsh elder. (Bot.)
   (a) The guelder-rose or cranberry tree (Viburnum Opulus).
   (b) In the United States, a composite shrub growing in salt
       marshes (Iva frutescens).

   Marsh five-finger. (Bot.) See Marsh cinquefoil (above).


   Marsh gas. (Chem.) See under Gas.

   Marsh grass (Bot.), a genus (Spartina) of coarse grasses
      growing in marshes; -- called also cord grass. The tall
      Spartina cynosuroides is not good for hay unless cut
      very young. The low Spartina juncea is a common
      component of salt hay.

   Marsh harrier (Zool.), a European hawk or harrier (Circus
      aeruginosus); -- called also marsh hawk, moor hawk,
      moor buzzard, puttock.

   Marsh hawk. (Zool.)
   (a) A hawk or harrier (Circus cyaneus), native of both
       America and Europe. The adults are bluish slate above,
       with a white rump. Called also hen harrier, and mouse
       hawk.
   (b) The marsh harrier.

   Marsh hen (Zool.), a rail; esp., Rallus elegans of
      fresh-water marshes, and Rallus longirostris of
      salt-water marshes.

   Marsh mallow (Bot.), a plant of the genus Althaea (
      Althaea officinalis) common in marshes near the
      seashore, and whose root is much used in medicine as a
      demulcent.

   Marsh marigold. (Bot.) See in the Vocabulary.

   Marsh pennywort (Bot.), any plant of the umbelliferous
      genus Hydrocotyle; low herbs with roundish leaves,
      growing in wet places; -- called also water pennywort.


   Marsh quail (Zool.), the meadow lark.

   Marsh rosemary (Bot.), a plant of the genus Statice
      (Statice Limonium), common in salt marshes. Its root is
      powerfully astringent, and is sometimes used in medicine.
      Called also sea lavender.

   Marsh samphire (Bot.), a plant (Salicornia herbacea)
      found along seacoasts. See Glasswort.

   Marsh St. John's-wort (Bot.), an American herb (Elodes
      Virginica) with small opposite leaves and flesh-colored
      flowers.

   Marsh tea. (Bot.). Same as Labrador tea.

   Marsh trefoil. (Bot.) Same as Buckbean.

   Marsh wren (Zool.), any species of small American wrens of
      the genus Cistothorus, and allied genera. They chiefly
      inhabit salt marshes.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Elder \El"der\, n. [OE. ellern, eller, AS. ellen, cf. LG.
   elloorn; perh. akin to OHG. holantar, holuntar, G. holunder;
   or perh. to E. alder, n.] (Bot.)
   A genus of shrubs (Sambucus) having broad umbels of white
   flowers, and small black or red berries.
   [1913 Webster]

   Note: The common North American species is Sambucus
         Canadensis; the common European species (S. nigra)
         forms a small tree. The red-berried elder is S.
         pubens. The berries are diaphoretic and aperient. The
         European elder (Sambucus nigra) is also called the
         elderberry, bourtree, Old World elder, black
         elder, and common elder.
         [1913 Webster + WordNet 1.5]

   Box elder. See under 1st Box.

   Dwarf elder. See Danewort.

   Elder tree. (Bot.) Same as Elder. --Shak.

   Marsh elder, the cranberry tree Viburnum Opulus).
      [1913 Webster]

WordNet (r) 3.0 (2006):

marsh elder
    n 1: any of various coarse shrubby plants of the genus Iva with
         small greenish flowers; common in moist areas (as coastal
         salt marshes) of eastern and central North America [syn:
         marsh elder, iva]