CLASSROOM Classroom In this section of Resonance, we invite readers to pose questions likely to be raised in a classroom situation. We may suggest strategies for dealing with them, or invite responses, or both. “Classroom” is equally a forum for raising broader issues and sharing personal experiences and viewpoints on matters related to teaching and learning science. Dilip Amritphale* and Seedlings of Dicots: Form and Function Santosh K Sharma Institute of Environment Management & Plant Sciences Seedlings of dicotyledons show a great diversity of form and Vikram University Ujjain (MP) 456010, India function. At least five seedling types based on position, *Email: [email protected] exposition and texture of cotyledons and 3 cotyledon types based on function are known. Understanding the relationship between the morphology and function of seedlings can provide an insight intothe regenerative strategies of species indifferent vegetation types and assist in constructing appropriate models for biodiversity management. Seedlings represent the final stage in the process of regeneration from seeds. There is no general agreement as to when the young plant developing from a seed ceases to be a seedling. One possible definition of a seedling is a young plant that contains at least some functioning structures produced from the initial seed reserves. This use of the term seedling is more liberal than that of ecologists who generally confine it to stages with the seed reserves still attached. But, it is more restricted than that used by most foresters who include individuals at a much later stage of development. Survival through the seedling stage, which is also the most Keywords Cotyledons, diversity, functional vulnerable phase in the life cycle of a plant, is critical for the morphology, seedling types. reproductive success of a species. Although the fact was known 468 RESONANCE May 2008 CLASSROOM for long, the relatively recent upsurge in the interest in seedling diversity coincided with the recognition of its importance in developing strategies for the conservation and management of biodiversity. The Classical scheme of classifying dicotyledonous seedlings in two groups viz.,epigeal and hypogeal is basedon the characteristics of germination and position of the cotyledons following germination. Seedlings having cotyledons raised above the soil level, borne on a long hypocotyl and getting exposed to light are epigeal. In hypogeal seedlings, the cotyledons are borne at or below soil level and normally remain together within the seed coat (Figure 1). Commonly available textbooks generally present a stereotypical description of epigeal and hypogeal seedlings that are heavily biased toward crop plants. For example, one would often come across drawings/images of epigeal seedlings of Cucumis sativus, Phaseolus vulgaris, Ricinus communis and similar other species showing cotyledons clearly raised above the soil layer by a well-developed hypocotyl. Likewise, depiction of typical hypogeal seedlings such as Cicer arietinum,Pisumsativum, Vicia faba and several other species showing bulky subterranean cotyledons and undeveloped hypocotyl is also fairly common. It is quite understandable since most crop seedlings can be raised relatively easily and are thus ideal classroom material. However, this kind of approach could also keep the students unaware of the Figure 1. Schematic dia- gram forepigealand hypo- geal seedlings. RESONANCE May 2008 469 CLASSROOM amazing seedling diversityobserved in natural plant communities, thus defeating the very purpose of studying biology. Students also need to appreciate that the classical scheme does not satisfactorily cover the range of variation in seedling morphology. Indeed, there are a number of cases known where seedlings cannotbe classified as epigeal or hypogeal following the criteria used in the classical scheme. For example, the seedling of Terminalia elliptica – a large deciduous tree of the family Combretaceae – is described as epigeal. Notably though, the green foliar Figure 2. Seedlings of (a) cotyledons in this species are raised above the ground not by the Terminalia elliptica (bar = growing hypocotyl, but by the long cotyledonary petioles 2.5cm) and (b) Zizyphus xylopyrus (bar = 1.0 cm). (Figure 2a). More or less similar type of seedling occurs in (Adapted from [3].) Zizyphus xylopyrus also (Figure 2b). Yet another instance where seedlings do not fit properly in the classical scheme is provided by Blumeodendron tokbrai. The seedlings in this species have cotyledons borne above the soil on an elongated hypocotyl, but they never leave the testa and thus remain unexposed to light. Moreover, the terms hypogeal and epigeal have been criticized on etymological ground also. It means, when germination occurs in or on the litter above the soil, it is rather incorrect to term the seedlings as hypogeal. In the same way, calling viviparous seedlings of mangroves as epigeal does not seem correct. Additionally, the classical scheme does not account for the fact that cotyledons could differ in exposition also i.e., they can be cryptocotylar (cotyledons enclosed within seed coverings) or phanerocotylar (cotyledons exposed i.e.,free fromseed coverings). Nancy Garwood (Southern Illinois University, USA) proposed a classification of dicotyledonous seedlings in 1996 that takes care of some of the inadequacies in the classical scheme. In her scheme, the seedlings are classified into the following five types (Figure 3) based on three characters of cotyledons viz., 1. position: epigeal or hypogeal, 2.exposition: cryptocotylar or phanerocotylar, 470 RESONANCE May 2008 CLASSROOM Figure 3. Schematic diagram for five seedling types. CHR: Cryptocotylar-hypogeal-reserve, PHR: Phanerocotylar-hypogeal-reserve, CER: Cryptocotylar-epigeal-reserve, PER: Phanerocotylar- epigeal-reserve, PEF: Phanerocotylar-epigeal-foliaceous. (Of the eight possible combinations, three types viz., cryptocotylar-hypogeal-foliaceous (CHF), cryptocotylar-epigeal-foliaceous (CEF) and phanerocotylar-hypogeal-foliaceous (PHF) have not been recorded/described so far) and 3. texture: fleshy (also termed as reserve) or foliaceous. Most commonly observed seedlings, particularly in tropical forests, are PEF type, whereas the least common are CER type. A few examples are given in Table 1. Although the classification can be successfully used to classify seedlings in different plant communities, there is ample scope for further improvement. This should not seem surprising given the incredibly vast array of seedling diversity in natural plant communities. Perhaps, following examples will clarify the point a little further. Table 1. Seedling Type Species 1. Cryptocotylar-hypogeal-reserve (CHR) type Mangifera indica, Quercus pubescens, Shorea robusta, Vicia faba 2. Phanerocotylar-hypogeal-reserve (PHR) type Cojoba arborea, Eugenia jambolana, Pithecellobium rufescens, Terminalia belerica 3. Cryptocotylar-epigeal-reserve (CER) type Durio excelsus, Omphalea oleifera, Rollinia salicifolia, Virola surinamensis 4. Phanerocotylar-epigeal-reserve (PER) type Acacia nilotica, Azadirachta indica, Phaseolus vulgaris, Tamarindus indica 5. Phanerocotylar-epigeal-foliaceous (PEF) type Anogeissus latifolia, Cucumis sativus, Manilkara hexandra, Terminalia arjuna RESONANCE May 2008 471 CLASSROOM In Peperomia peruviana, a member of Piperaceae, the seedlings are hetercotylar i.e., the cotyledons are unequal in size and dissimilar in function (Figure 4a). One of the two cotyledons remains at the soil level within the seed coverings acting like a haustorium (hypogeal-crypto- cotylar-reserve), whereas the other is withdrawn fromthe seed coverings and becomes the first aerial assimilating organ (epigeal-phane- rocotylar-foliaceous). Needless to say, it is rather difficult to fit P. peruviana seedlings into any one of the five seedling types described earlier above. Somewhat analogous condition exists in Artocarpus integer (Moraceae), which is a large tree native to the tropical forests of south-east Asia. The cotyledons are borne at soil level and are unequal in size (Figure 4b).Duringgermination, Figure 4. Seedlings of (a) the larger cotyledon remains at the soil level within the seed coat Peperomia peruviana (not (i.e., it hypogeal, cryptocotylar). On the other hand, the smaller to scale), (b) Artocarpus one opens like a lid in the air with its upper surface completely integer, (c) Barringtonia exposed (i.e., it tends toward epigeal condition, semi- racemosa, and (d) Rhizo- phora mucronata (bar = 2.5 cryptocotylar). Again, it is rather tricky to accommodate acotylar cm for b, c and 5.0 cm for seedlings (cotyledons absent or reduced to scale-like structures) d). such as that observed, for example, in Barringtonia racemosa (Figure4badaptedfrom [3]; Fig- (Lecythidaceae). Here the embryo is essentially a massive food- ures 4(a) and (c) from [4]) storing hypocotyl (Figure 4c) that persists long after germination supporting the growth of the seedling. What is more, it is difficult yet to satisfactorily classify viviparous seedlings of several mangrove species such as Bruguiera gymnorrhiza, Ceriops tagal, and Rhizophora mucronata (Figure 4d). 472 RESONANCE May 2008 CLASSROOM The relationship between the seedling form and function is often not apparent. Indeed, this could have been one of the reasons for the slower development of investigations into the functional significance of the morphological diversity. Presently, considerable emphasis is placed on the functional aspects of seedling