Stems of plant have to be rigid enough to remain upright. At the same time, they have to be flexible to withstand the bending forces of strong winds.
In a herbaceous dicotyledonous stem, collenchyma forms a fairly rigid strengthening outer hollow cylinder. Together with the turgid parenchyma in the cortex and pitch, it keeps the stem upright. The vascular bundles with their pericyclic fibre 'caps' form strengthening solid rods which are arranged in a ring around the pitch. The arrangement gives flexibility and resilience to the stem, allowing it to bend back and forth without breaking when subject to the force of strong winds. The scattered arrangement of vascular bundles in a monocotyledonous stem also gives flexibility to the stem.
In a tree trunk, support is mainly due to secondary xylem, which forms a solid central cylinder. There is only a thin outer cylinder kg living tissues around the xylem cylinder. This arrangement supports the large structure of a tree. A tree trunk, however, has little flexibility. As a result , during a very strong wind, large woody trees tend to be uprooted, while young plants remain rooted.
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