At a primordial developmental stage foliage leaves in angiosperms are subdivided into a proximal leaf base and a distal leaf part. The distal part (Oberblatt) was recently named hyperphyll (see Weberling 1989: 45); similarly, the proximal part (Unterblatt) can be called a hypophyll. In many monocotyledons the hypophyll tends to be much more extended and contributes a far greater part to the leaf than in dicotyledons, but there are several exceptions to this rule.
The discussion on the interpretation of the foliage leaf of monocotyledons is dominated by the so-called phyllode theory, which goes back to de Candolle (1828) and was extensively supported by Arber (1918 ff.). According to this theory, the unifacial, often more or less cylindrical or ensi-
form hyperphyll in monocotyledons represents only a petiole; the lamina is said to be totally missing (Fig. 3D,H). This view was refuted by Gaisberg (1922) andPeters (1927); more recently, the extensive and careful investigations of Kaplan (1972, 1973, 1975) have demonstrated that the unifacial hyperphyll in monocotyledonous leaves is equivalent to the complete hyperphyll of dicotyledons (Fig. 4). Thus, the term phyllode is unsuitable for monocot leaves. Recent investigations by Bharathan (1996) additionally demonstrate that the hyperphyll region may differentiate into an extended bifacial lamina and a unifacial tip. Thus it is not appropriate to consider automatically a unifacial leaf tip (Vorlauferspitze, precursory tip) as equivalent to the complete hyperphyll, and the rest of the leaf, whether or not it is differentiated into sheath, petiole, and lamina, as representing the hypophyll. In many cases, an adequate interpretation will be possible only after careful ontogenetic investigations (Bloedel and Hirsch 1979). Therefore it is preferable to use the terms petiole and lamina in a descriptive sense only to refer to a narrowed and a broadened leaf area, respectively.
In Poaceae and Commelinaceae we never find a unifacial leaf tip. On the other hand, in these families the leaves are clearly subdivided into a culm-embracing sheath and a bifacial blade, which itself may be separated from the sheath by a shorter or longer petiole (Fig. 51). Arber (1922a, 1923a) considered the blades in both families to be phyllodes, despite their bifacial structure and a normal bundle orientation. Such an imposition of a theory onto nature brings no advantage at all. Wherever a clearly distinguishable petiole and lamina are found, such as in Alismataceae, palms, Diosco-reaceae, or bamboos, these organs should be called petiole and lamina. Terms like pseudopetiole and pseudolamina are dispensable.
Where the leaves consist of a more or less linear, parallel-veined hypophyll and a hyperphyllary tip (Fig. 5J-N), the relative contributions of the two parts to the adult foliage leaf varies considerably (see Fig. 4A,C). Leaf parts formed by the hypophyll are always bifacial, whereas the hyperphyll may be completely or in part unifacial. The extent to which each of the two leaf zones and the unfacial and bifacial regions can contribute to the construction of the adult leaf is shown in Fig. 4A-F.
The leaf base in monocotyledons only rarely bears paired appendages, which usually are termed stipules. However, true stipules, as defined by Weberling (1958,1975), seem to be absent. Lateral appendages as found in some Smilax species are vaginal lobes rather than stipules. Median, often hyaline appendages of the leaf base (as in Potamogetonaceae, Poaceae, Zingiberaceae) are called ligules and can be interpreted as median vaginal lobes.
Small, few-celled appendages are regularly found in the axils of the Alismatidae, where they were discovered by Irmisch (1858a,b), who named them squamulae intravaginales. These supposedly glandular organs were studied later by Gibson (1905), Arber (1923a,b) and Tomlinson (1982). Outside the Alismatidae, similar squamules are reported from some Araceae (Philodendroti, Cryptocoryne, Lagenandra, see Dahlgren et al. 1985).
An appendage specific to some palmate and costapalmate palm leaves is the hastula, which is found at the junction between petiole and lamina, and which may be developed adaxially and abaxially as well (Fig. 5P,Q).
Palm leaves are pinnate, palmate, costapalmate, or, in Caryota, twice-pinnate. Developmentally, the blade is always initially undivided, but early in ontogeny the lamina exhibits a regular plication. The separation of the pleats occurs late in ontogeny. The process of blade plication and subsequent separation of the leaflets was described in detail by Kaplan et al. (1982a,b) and Dengler et al. (1982).
In Araceae the leaf shape is exceedingly diverse (see Mayo et al. 1997). It ranges from simple elliptic to highly dissected, as in Dracontium, Taccarum, or Amorphophallus. Fenestrate (perforated) leaves are a special peculiarity of the family; they can be found, e.g., in species of Monstera, Dracontium, and Amydrium. These perforations are of secondary origin, since the respective leaves initially have an entire lamina, in which certain areas undergo necrosis during early ontogenetic development (Arber 1925a, Hotta 1971). If the holes extend to the margin of the lamina, the leaf
Fig. 2A-L. Monocotyledonous growth forms. A Socratea exorrhiza, unbranched palm with prop roots. B Euterpe oleracea, basally branched palm. C Cordyline kaspar, richly branched tree with secondary thickening. D Abromeitiella sp., schematic section through a cushion. The oldest parts decaying, the younger shoots rooting in the organic matter. Inflorescences indicated as circles. E Coelogyne salmoni-color, epiphytic orchid with specialized storage axes. F Taeniophyllum sp., epiphytic orchid with much reduced shoots, assimilating roots (r), and a small inflorescence (in). G Lemna gibba, free-floating plant with much reduced vegetative shoot system. H Pandanus sp., richly branched shrub with strong stilt roots. I Zingiber zerumbet, vegetative leafy shoots strongly different from scale-leaved flowering shoots. J Monstera dubia, scale-leaved seedling axis growing skototropically along soil surface until reaching a trunk; when climbing upwards, producing small shingle leaves and finally foliage leaves typical of the adult plant. K Schematic representation of regular sympodial rhizomatous growth, each sympo-dial element ending in an inflorescence. This growth type is very common in monocotyledons. L Scilla verna, a bulbous member of Hyacinthaceae. Some figures modified. Not to scale. (A, B Kahn and Granville 1992; C Tomlinson and Fisher 1971; D Rauh 1990; E Bechtel et al. 1993; F Strasburger, Lehrbuch der Botanik, 32. Aufl. 1992; G Hegelmaier 1868; H Brooks 1993; I Wagner et al. 1990; J Bown 1988; K Holttum 1955; L Dahlgren et al. 1985)
becomes pinnatisect. Truly pinnate leaves typical of many dicotyledons are missing in monocotyledons, although the lamina of Zamioculcas resembles a truly pinnate lamina.
Leaf tendrils are developed only very sporadically in monocotyledons. They are formed by the apices of the lamina in Ghriosa and Flagellaria. In Smilax they are paired outgrowths of the leaf base of an obscure nature (Fig. 5F).
In the Marantaceae the junction between petiole and lamina is a swollen pulvinus or geniculum, which turns the lamina into a favorable position to light. A pulvinus is also found in some Zingiberaceae and Araceae. Curiously enough, in Anthurium oerstedianum and some other Araceae, the pulvinus has its position on the petiole halfway between its base and the lamina.
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