Seedlings of monocotyledons, without any exception, possess only 1 cotyledon. This cotyledon represents 1 leaf. There is no morphological or developmental indication of either syncotyly or anisocotyly (Tillich 1992). The cotyledon is sub-
Fig. 4A- F. Modes of participation of leaf base (shaded) and hyperphyll (white) in the leaf construction of monocotyledons. A Unifacial hyperphyll (cylindrical or ensiform), not differentiated in petiole and lamina (e.g., /uncus spp.). B Hyperphyll with unifacial petiole, bifacial lamina, and unifacial tip (e.g., Orontium aquattcum). C Leaf base ribbonlike elongated, the hyperphyll a small unifacial tip (e.g., Dracaena deremensis). D Leaf base differentiated in sheathing base, petiole, and lamina, hyperphyll a small unifacial tip (e.g., Hosta spp.). E Hyperphyll differentiated in bifacial petiole, lamina, and a unifacial tip (e.g., Dioscorea spp.). F Hyperphyll completely bifacial (e.g., Croomia japonica). (Following Troll 1955)
divided, like any foliage leaf, into a leaf base (hypophyll) and an upper part (hyperphyll).
The cotyledon base embraces the seedling axis and is often tubular. The assumed ancestral form is a short sheath without or with inconspicuous appendages. In the simplest case, the hyperphyll may have only a haustorial function, i.e., it is completely hidden in the seed (Fig. 6A). Seedlings with compact cotyledons in most cases develop 1-several scalelike cataphylls as their first plumular phyllomes. From this basic model a great diversity of forms has evolved (Figs. 6,7).
A first evolutionary line leads to an elongated, green, upright cotyledonary hyperphyll capable of assimilation (Fig. 6B). The green hyperphyll is usually cylindrical, but in Trilliaceae it is distally flattened to form a small lamina. Except for some bulb or tuber-bearing species this cotyledon type is generally combined with green eophylls. The same is true for the third type, which is characterized by a well-developed coleoptile instead of an elongated hyperphyll. This organ is a secondary outgrowth from the margin of a tubular cotyledonary sheath (Fig. 6C). TTie elongation growth of the sheath tube is less the more strongly the coleoptile develops. At the most derived stage the sheath tube is totally reduced and the hyperphyll seems to arise directly from the base of the coleoptile, as in Poaceae, Cyperaceae, or Marantaceae (Figs. 6D, 8J). The coleoptile seems to protect the first eophylls.
Fig. 5A-Q. Foliage leaves in monocotyledons. A-I Laminated leaves. A Dioscorea bulbifera. B Eichhortiia crassipes. C Streptolirion volubile. D Arum maculatum. E Sagittaria montevidetisis. F Smilax perfoliata. G Monstera deliciosa. H Proiphys amboinensis. I Sinobambusa kunishii. J Sansevieria trifasciata with "typical", parallel-veined monocotyledonous leaves. K-N Leaves with unifacial apices. K Sansevieria trifasciata. L Chlorophytum comosum. M Dracaena fragrans. N Dracaena deremensis. O-Q Leaves with appendages. O Anthoxanthum formosanum with well-developed ligule. P, Q Sabal sp., insertion of lamina on petiole. P Upper face, with prominent adaxial hastula. Q Lower face, with inconspicuous ubaxial hastula. Some figures modified. Not to scale, h hastula, I tendril, vl vaginal lobes. (A Liu and Huang 1978; B after Weber 1950; C, D Arber 1922a, 1925a; E Cabrera 1968; F Koyama 1978; G Rohweder and Endress 1983; H Telford 1987; I, O Hsu 1978; J Dahlgren et al. 1985; K-N Troll 1955, P, Q Tomlinson 1961)
The next cotyledon type again has assimilatory capacity, but here this is achieved by a dilatation of the cotyledonary sheath to form a laminalike urea, while the hyperphyll portion remains in the seed and has only a haustorial function (Fig.. 6E, I''). This cotyledon type is typically found in Costaceae, in some Araceae (Philodendron, Colocasia, see Tillich 1985), in Bromeliaceae-I'itcairnioideae, and in Xyris.
The last type to be reviewed here is typical of species, whose seeds contain no or only remnants of endosperm, so that nutrients have to be stored in the cotyledon. In these cases, the storage organ is generally the cotyledonary hyperphyll (Fig. 6G). Outside the Alismatidae the storage cotyledon is known only in several Araceae and in Cyanastrum (Tillich 1985, 1995b).
Some special features are exhibited by palm seedlings. The cotyledon never produces chlorophyll even if developing in permanent light. It may be of the compact type with no appendages or have a very short coleoptile (Cocos, Roystonea, Salacca). In other cases, the hyperphyll elongates considerably and the sheath is either tubular (Phoenix) or completely absent, but with a long coleoptile (Sabal). The first plumular leaves are aleays cataphylls (Tomlinson 1960). Martius (1823) called palm seedlings with compact cotyledons, admotive, and those with an elongated hyperphyll, remotive.
The hypocotyl in monocotyledonous seedlings is mosdy inconspicuous. However, in a few cases it is well developed and may even become a small tuber (Pinellia, Paris, Trillium, some Discorea). In most families of the Alismatanae it is an important
Fig. 6A-G. Cotyledon types in monocotyledons. A Sansevi-eria grandis, compact cotyledon. B Acorus calamus, cotyledon with elongated, assimilating hyperphyll. C Bulbine semi-barbata> three stages of coleoptile development. D Sarco-phrynium brachystachys, strongly developed coleoptile, the numerous collar roots growing and branching vigorously unlike the vestigial primary root. E, F Cotyledons with expanded, assimilating sheath. E Dyckia sulphurea. F Costus megalobractea, the cotyledon consisting of a sheathing base, a laminalike portion, and a minute unifácial haustorium (hidden in the background). G Aponogeton distachyus, storage cotyledon,* tuber formation commencing with a swelling of the epicotyl. co Cotyledon; cor collar roots; cp coleoptile; cs cotyle-donary sheath; ep epicotyl; h haustorium; hy hypocotyl; o opening of cotyledonary sheath or coleoptile; pi eophyll; pr primary root; sr shoot born root additional storage organ of the appropriately named macropodous embryo (Fig. 8F,H).
The transition zone between the hypocotyl and the primary root is the collar, actually the lowermost part of the hypocotyl. A distinguishing feature of the collar is the development of dense rhizoids (Fig. 8F,H)- Roots formed endogenously at the collar level are called collar roots (Grenzwurzeln).
The primary root is initiated exogenously, mostly exactly at the root pole of the embryo, i.e„ at the attachment point of the suspensor. Lilaeay Triglochin, Ruppia, and Aponogeton are exceptions, and here the primary root is initiated at
Fig. 7. Modes of evolutionary specialization of cotyledon structure in monocotyledons. Sheath region (hypophyll) shaded. Explanation in the text
Fig. 7. Modes of evolutionary specialization of cotyledon structure in monocotyledons. Sheath region (hypophyll) shaded. Explanation in the text some distance from the root pole (Yamashita 1970, 1972, 1976).
Shoot born roots are always initiated endogenously. When a root develops in a young parental tissue, as is the case in seedlings, it may stimulate
the peripheral tissue layers of its parental organ to form a kind of bag over the growing root apex. Such a covering bag, which is later penetrated by the growing root, is called the coleorhiza (Fig. 8C,D,E).
The primary root in monocotyledons is often much reduced, although in Strelitzia or several Convallariaceae it grows and branches vigorously. Usually, it grows only weakly and is short-lived (Figs. 6D, 8H). In some Tillandsia spp. it is only a small rudiment (Fig. 81), in Marantaceae it may fail to commence elongation growth, and the primary root is not initiated al all in Poaceae, Zostera, l.emnaceae, and Pistia (Fig. 8K).
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