This has the same functions as those of a dicotyledon; therefore the cell types and tissues are similar. However, the arrangement of the tissues does differ because increase in diameter by secondary growth does not take place. The stem relies on extensive sclerenchyma tissue for support that, in the maize stem shown in Figure 6.4, is found as a sheath around each of the scattered vascular bundles. Monocotyledonous stem structures are seen at their most complex in the palm family. From the outside, the trunk would appear to be made of wood, but an internal investigation shows that the stem is a mass of sclerified vascular bundles. The absence of secondary growth in the vascular bundles makes the presence of cambium tissue unnecessary.
Secondary thickening is found not only in trees and shrubs, but also in many herbaceous perennials and annuals that have woody stems. However, trees and shrubs do exhibit this feature to the greatest extent.
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