The Collaborative International Dictionary of English v.0.48:

Peppergrass \Pep"per*grass`\, n. (Bot.)
   (a) Any herb of the cruciferous genus Lepidium, especially
       the garden peppergrass, or garden cress, Lepidium
       sativum; -- called also pepperwort. All the species
       have a pungent flavor.
   (b) The common pillwort of Europe (Pilularia globulifera).
       See Pillwort.
       [1913 Webster]

WordNet (r) 3.0 (2006):

Pilularia globulifera
    n 1: European water fern found around margins of bodies of water
         or in wet acid soil having small globose sporocarps [syn:
         pillwort, Pilularia globulifera]