The Collaborative International Dictionary of English v.0.48:

Leucophyll \Leu"co*phyll\ (l[=u]"k[-o]*f[i^]l), n. [Leuco- + Gr.
   fy`llon a leaf.] (Chem.)
   A colorless substance isomeric with chlorophyll, contained in
   parts of plants capable of becoming green. --Watts.
   [1913 Webster]