Auxin is a growth type of this substance; it regulates plant growth, especially in stems
The result 
is a kind of lopsided growth in the shoot, which causes the vine to coil around ... 
especiallyproduce ethylene, a gaseous plant hormone, one effect of which is to
... [DOC] [TLE] auxins - Memidex dictionary/thesaurus [PAR] "auxins" is a plural form of "auxin": a plant hormone that promotes root formation 
and ... substance noun (substance and materials) ... plant hormone that regulates 
the amount, type, and direction of plant growth.