The great majority of the monocotyledons lack a cambium. This is, according to Holttum (1955), the basic limitation for the construction of the shoot system. Since there is no cambium in the primary root, it soon becomes too small in diameter to conduct the nutrients and water needed for the growth of the shoot system. Thus it is necessary to produce shoot-borne roots early during the seedling stage and establishment growth (Tomlinson and Esler 1973). At the same time, the axis undergoes a marked primary thickening, increasing from internode to internode and leading to the typical inverted conical shape of the basal primary axis (Fig. 1). The number of leaves per node is generally restricted to 1, since the leaf base in monocotyledons usually embraces the axis for more than half the circumference and there remains no space for further leaf primordia at a given node (see Tomlinson 1970b).
The limited conductive capacity of the axis is ulso responsible for the restricted branching of upright aerial shoots. The development of tall trees is possible only since the number of leaves in a given individual always remains constant, as is I rue e.g., for palms or Strelitzia nicolai. Vegetative branching, if happening at all, mainly takes place from basal nodes.
Bearing these limitations in mind, it is fascinating to see what a great diversity of growth forms and special adaptations is to be found in monocotyledons. A selection is shown in Fig. 2. The most remarkable specialized forms include the epiphytic orchids and bromeliads, the submarine members of Alismatales and Najadales, the much-reduced Lemnaceae, and the mycotrophic Bur-manniales and Triuridales. In some climbing members of the araceous genera Monstera and Pothos, a peculiar skototropic growth (Strong and Ray 1975, Boyce and Poulsen 1994, Boyce and Nguyen Van Dzu 1995; Fig. 2J) helps the young plant to find a host tree. Among the rattan palms, the genus Calamus includes species with shoots more than 180 m long, the longest known in the plant kingdom (Dransfield 1978). Despite this great diversity, it is surprising that the monocotyledons contain only a very limited number of therophytes. They are mainly found in Poales and Cyperales; elsewhere they are very scattered, e.g., in Asparagales (Bulbinella, Asphodelus p.p.), Pontederiaceae, or Commelinaceae.
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