PHYLLODE THEORY

            Modification of petiole or any part of the rachis (axis of a compound leaf) into leaf-like flattened green structure is known as ‘phyllode’. The phyllode is a characteristic of xerophytic plants, and its main function is to carry on photosynthesis.

            The leaves of many plants which have to perform specialized functions become modified or metamorphosed into distinct forms. The phyllodes are common in monocotyledons, but rare in dicotyledons.

            The leaves of most monocotyledons are phyllodes instead of typical leaves in true morphological sense. The leaves of those plants lack the lamina or blade. They are so called apparent leaf blade and comprises actually expanded petiole or leaf base or both. This replacement of leaf lamina by phyllode is not only confined to monocotyledons but also occur rarely in dicotyledons, e.g., Acacia, Eryngium, Oreomyrrhis, etc. This view regarding the presence of phyllode in majority of the families of monocotyledons is supported by Arber (1920, 1925) and is known as Phyllode theory.

     The peculiarity of possessing basal leaf sheath and a simple blade by numerous monocotyledonous families is a common feature. Phyllodes found in monocotyledons may be either petiolar or a leaf sheath in origin, the true lamina being absent or suppressed. So there are two types of phyllode – the petiolar phyllode and the leaf sheath phyllode.

            1. The petiolar phyllode:- The cylindrical elongated leaf of Allium sepa and other species of Allium is a typical petiolar phyllode. In this case, the membranous sheath is very distinct and apparent which is prolonged upward into a green cylindrical petiole forming a lamina. In Trigochin maritinum, the same morphological structure is found. Here the leaf is actually composed of basal sheath and awl-like petiole. The petiole may be flattened or expanded in vertical as well as in horizontal plane.

            2. Leaf sheath phyllode:- Leaf sheath and leaf base phyllodes also occur in many monocotyledons. In a leaf sheath phyllode the leaf is further reduced. In petiolar phyllode, the leaf represents two parts – the base and the petiole. The lamina is suppressed whereas the leaf sheath phyllode consists of flattened base only. In several genera of Iridaceae and Liliaceae, the petiole is very small or reduced to minute apical portion, hence the leaf consists of flattened base only. The leaf sheath phyllode is found in monocotyledons like Doryanthes, Distichia, Elegia, etc.

            In dicotyledons, the petiole metamorphosed into expanded blade-like structure. In Australian Acacia, the petiole or any part of the rachis become flattened or winged taking the shape of the leaf and turning green in colour. This flattened or winged petiole or rachis is known as phyllode. There are altogether 300 species of Australian Acacia, all showing phyllode. Examples are – A. moniliformis, Acacia melanoxylon, etc.

            SIGINFICANCE OF PHYLLODE THEORY

            1. Morphological support of this theory is given by close resemblance of the monocotyledon leaf to the sheathing bases of some dicotyledonous leaves.

            2. The theory holds that the typical monocotyledon leaf arose by the self-adaptation of dicotyledon leaf.

***********