Figure 7.30 Distribution of growth and wall extension at four positions along a cucumber hypocotyl. (a) Growth rate is most rapid near the hook. (b) Hypocotyl segments were frozen, thawed, abraded and stretched under a 20 g load in an acidic buffer (pH 4.5), which is much less tension than exerted by $P$. Most rapid extension occurred in the fast-growing hook. (c) When measured at pH 6.8, segments extended very little. (d) Segments in which enzymes were denatured by boiling did not extend under the load. (SJ McQueen-Mason, J Exp Bot 46: 1639-1650, 1995)