(a) Alphonse de Candolle classified plants of economic importance into various classes. List these classes and state two examples…

(a) De Candolle's classes of economic plants

Alphonse de Candolle (Origine des plantes cultivées, 1882) grouped cultivated plants by region of origin and age of cultivation, into six classes:

1. Old World plants cultivated for more than 4,000 years, e.g. wheat, rice.
2. Old World plants cultivated for about 2,000 years or more, e.g. oat, sugarcane.
3. Old World plants brought into cultivation more recently, e.g. coffee, okra.
4. New World plants cultivated before the European discovery of America, e.g. maize, sweet potato.
5. New World plants of pre-Columbian cultivation with less antiquity, e.g. potato, pineapple.
6. New World plants brought into cultivation after Columbus, e.g. cinchona, rubber.

Classes 1–3 are Old World and classes 4–6 New World; the assignment of individual crops to a class varies slightly between textbooks.

(b) Anatomy of C₃ and C₄ leaves

C₃ plants: bundle sheath cells are small, thin-walled and have few or no chloroplasts, so they play little part in photosynthesis. Mesophyll cells are differentiated into palisade and spongy parenchyma, all with granal chloroplasts, and they carry out the whole Calvin cycle.

C₄ plants: show Kranz anatomy. Large, often thick-walled bundle sheath cells with many chloroplasts form a wreath around each vein, and have few intercellular spaces and a suberised lamella in some species, which reduces CO₂ leakage. Their chloroplasts are large, rich in starch and Rubisco, and in many species (e.g. maize, sugarcane) agranal or with reduced grana. The mesophyll cells are arranged radially around the sheath, have granal chloroplasts and do not form a palisade layer; they fix CO₂ by PEP carboxylase into C₄ acids.

(c) Trichomes and their systematic value

Trichomes are epidermal outgrowths. Non-glandular (covering) trichomes may be unicellular or multicellular, simple, branched, stellate, peltate or T-shaped, e.g. cotton seed hairs (unicellular), stellate hairs in Malvaceae and Solanum, and scale-like peltate hairs in Elaeagnus. Glandular trichomes secrete oils, resins, mucilage or nectar, e.g. capitate glands in Mentha and Ocimum, and the digestive glands of Drosera. Stinging hairs of Urtica have a silica-tipped cell holding irritant fluid. Root hairs absorb water.

Systematic value: the type, cellularity, branching and distribution of trichomes are fairly stable within a taxon and are used to distinguish genera and species, e.g. stellate hairs in Malvaceae and Brassicaceae, and they are also used in identifying herbarium material and in forensic and drug-adulteration work.

(d) Vegetable fibres (excluding wood) by botanical origin

• Seed fibres: from seed-coat hairs, e.g. cotton (Gossypium), and kapok (Ceiba pentandra) from the fruit wall hairs.
• Bast (stem) fibres: from phloem or pericyclic fibres of dicot stems, e.g. jute (Corchorus), flax (Linum), hemp (Cannabis), ramie (Boehmeria).
• Leaf fibres: from vascular bundles of monocot leaves, e.g. sisal (Agave sisalana), pineapple (Ananas), and abacá (Musa textilis, leaf-sheath fibre).
• Fruit fibres: from the mesocarp, e.g. coir from coconut (Cocos nucifera).
• Grass and other fibres: stems of Saccharum (munj) and Bambusa, used for ropes and paper.

(e) Isolation of single cells and their viability

Isolation: (1) Mechanical method: gentle maceration or homogenisation of soft tissue, such as leaf mesophyll, in a buffered medium, followed by filtration and centrifugation; it is quick but cells may be injured. (2) Enzymatic method: tissue is treated with macerozyme (pectinase) to dissolve the middle lamella, which releases intact cells with walls; adding cellulase would remove the wall and give protoplasts.

Viability tests: Evans blue is excluded by living cells, but dead cells stain blue. Fluorescein diacetate (FDA) is hydrolysed by esterases in living cells to fluorescent fluorescein, seen as green under UV or blue light. TTC is reduced to red formazan by active dehydrogenases. Growth and plating efficiency on culture medium confirm viability.