Concept Explanation
A legume is the distinctive seed pod of beans, peas, and their relatives. It's a specialized fruit that splits open along two seams when ripe, releasing the seeds inside.
Key Characteristics
Structure:
- Develops from a single carpel (ovary)
- Contains multiple seeds in a row
- Two seams (sutures): dorsal and ventral
- Usually dry when mature
- Splits open (dehisces) along both seams
Types:
- Typical: Splits along both seams (beans, peas)
- Indehiscent: Doesn't split open (peanut, carob)
- Loment: Breaks into segments between seeds
The Fabaceae Family
Legumes define the Fabaceae (formerly Leguminosae), one of the most important plant families:
Common Names:
- Legume family
- Pea family
- Bean family
Diversity:
- 3rd largest plant family
- ~19,000 species
- Trees, shrubs, vines, herbs
- Found worldwide
Economic Importance:
- Food crops: beans, peas, lentils, soybeans, peanuts
- Timber trees: rosewood, cocobolo
- Ornamentals: wisteria, sweet pea
- Forage crops: clover, alfalfa
Special Adaptations
Nitrogen Fixation: Most legumes have root nodules containing bacteria (Rhizobium) that convert atmospheric nitrogen to plant-usable forms. This makes legumes:
- Soil improvers
- Important in sustainable agriculture
- Pioneer species in succession
- Green manure crops
Why It Matters:
- Reduces need for fertilizer
- Enriches degraded soils
- Supports ecosystem recovery
- Carbon-negative agriculture
Costa Rican Legume Trees
Costa Rica has many important legume trees:
Native Species:
- Guanacaste (Enterolobium cyclocarpum): National tree, ear-shaped pods
- Poró (Erythrina species): Living fences, coral-tree flowers
- Guapinol (Hymenaea courbaril): Resin-filled pods
- Cenízaro (Albizia saman): Rain tree, livestock fodder
Introduced Species:
- Leucaena (Leucaena leucocephala): Fast-growing, can be invasive
- Acacia species: Various uses, some invasive
Identification Tips
Look For:
- Compound leaves (usually pinnate or bipinnate)
- Pea-like flowers (in many species)
- Characteristic pod fruits
- Often nitrogen-fixing (lush green leaves)
Flower Types: Three subfamilies with different flowers:
- Papilionoid: Pea-like (sweet pea, beans)
- Caesalpinioid: Irregular but not pea-like (poinciana)
- Mimosoid: Pom-pom clusters (mimosa, acacia)
Uses and Importance
Food:
- Beans, peas, lentils, soybeans
- Carob, tamarind
- Protein-rich seeds
Timber:
- Cocobolo, rosewood
- Hardwoods with beautiful grain
- Often CITES-protected
Agroforestry:
- Living fences (Poró, Madero Negro)
- Coffee shade trees
- Fodder trees for livestock
- Soil improvement
Ecological:
- Pioneer species
- Nitrogen fixation
- Wildlife food (nectar, seeds, pods)
- Erosion control
Pod Variations
Legume pods show amazing diversity:
By Size:
- Tiny: Some Acacia (< 5 cm)
- Huge: Enterolobium (up to 15 cm diameter)
By Shape:
- Straight: Most beans
- Coiled: Alfalfa
- Winged: Some Pterocarpus
- Ear-shaped: Guanacaste
By Function:
- Wind-dispersed: Flat, papery
- Water-dispersed: Buoyant
- Animal-dispersed: Edible pulp
- Explosive: Spring-open violently
Cultural Significance
Traditional Uses:
- Food security crops worldwide
- Traditional medicine
- Dyes and tannins
- Construction wood
Modern Agriculture:
- Crop rotation with legumes
- Green manure/cover crops
- Intercropping systems
- Organic farming staples
Conservation Note
Many legume trees are:
- Overharvested for timber (rosewood, cocobolo)
- CITES-listed species
- Important restoration species
- Used in reforestation projects
Balance:
- High value timber = conservation pressure
- Nitrogen-fixing = restoration value
- Need sustainable management