Ave Maria, Ave Maris Stella is the second of Langlais' "Trois paraphrases grégoriennes". It's built on two gregorian hymns : Ave Maria and Ave Maris Stella (their transcription is attached below) :
a- first we hear the first part of the Ave Maria both at hands (voix celeste) and pedal (flute 4) in longer values at pedal ;
b- at 1'50, the atmosphere becomes agitated (prayer of the tormented humans, all flues 8 feet) ; this section is built on the second part of the Ave Maria, which is a kind of litany ;
c- at 2'50, the atmosphere becomes more serene and we hear the previous litany on the hands (flutes 8,4) but combined with the Ave Maris stella at the pedal (cromorne) ; this could appear as the Virgins 's answer to the men's prayer ;
d- at 3'45, there a short transition with joyful imitations on the Ave Maris Stella theme (flutes 8) ;
e- at 4'20, the peaceful and serene last part begins and the Ave Maria comes back. LH : voix céleste - RH : bourdons 8 (and not the usual flute harmonique).
Messiaen, who was Langlais' great friend, wrote about this piece : "Here is quite a pretty organ piece, very poetic, well written for the instrument and easy to play". I respectfully share this opinion, but for the last part : this piece is surely not virtuosic at all, but very awkward to learn with a dense writing, uncomfortable hands and body positions and many many many substitutions ; but well worth the effort !