Very, very similar, but not quite. Siehe: http://de.wikipedia.org/wiki/Blattform#Ansatz...stängelumfassend
(amplexikaul): Die Basis der Spreite greift um (fast) den ganzen Stängel = English clasping
-- the leaf mostly surrounds the stem but doesn't quite meet on the opposite side of the stem unless it's further down the stem from the bladedurchwachsen
(perfoliat): Das Blatt umgibt den Stängel geschlossen = English perfoliate
-- the leaf completely surrounds the stem so that the stem seems to grow through the leaf blade
(If 2 leaves on opposite sides of the stem grow together at their bases, then it's verwachsen
At least those are the technical definitions. In actual practice, I see by your Hellerkraut example that they are often used interchangeably. EDIT: Also in English, as in the English common names of this plant, clasping ... & perfoliate ...