lateral growth diffuse secondary thickening Occurs in the stems (PO:0009047) of some Arecaceae (palms). dhl 2012-12-06T15:57:50Z GO:0080193 biological_process Lateral growth of the older parts of a stem that occurs when the central parenchyma cells and the not yet fully differentiated fiber cells of the bundle sheaths continue to undergo cell division and expansion for a long period of time, leading to an increase in girth of the stem.