The coffee plant is a woody perennial evergreen dicotyledon that belongs to the Rububiaceae family. It has a main vertical trunk (orthotropic) and primary, secondary, and tertiary horizontal branches (plagiotropic). Two main species of coffee are cultivated today. Coffea Arabica known as Arabica coffee accounts for 75-80% of the world’s production. Coffee canephora, known as Robusta coffee, is more robust than Arabica plants, but produces an inferior tasting beverage with a higher caffeine content. The coffee plant can grow to heights of 10 meters if not pruned, but producing countries will maintain the coffee at a height reasonable for easy harvesting.
Three to four years after the coffee is planted, sweetly smelling flowers grow in clusters in the axils of the leaves. Fruit is produced only in the new tissue. The Arabica species is self-pollinating, whereas the Robusta species depends on cross pollination. About 6-8 weeks after the flowers are fertilized, cell division occurs and the fruit remains as a pin head for a period that is dependent upon the climate. The ovaries will then develop into drupes in a rapid growth period that takes about 15 weeks after flowering. During this time the integument takes on the shape of the final bean. After the rapid growth period the integument and parchment are fully grown and will not increase in size. The endosperm remains small until about 12 weeks after flowering. At this time it will suppress, consume, and replace the integument. The remnants of the integument are what make up the silverskin. The endosperm will have completely filled the cavity made by the integument nineteen weeks after flowing. The endosperm is now white and moist, but will gain dry matter during the next several months. During this time the endosperm attracts more than seventy percent of the total photsynthesates produced by the tree. The mesocarps will expand to form the sweet pulp that surrounds the bean. The cherry will change color from green to red about thirty to thirty-five weeks after flowing.
The root system can extend 20-25km in total length (Malavolta, 195) and the absorbing surface of a tree ranges from 400 to 500m2 (Nutman). There are main vertical roots, tap roots, and lateral roots which grow parallel to the ground. The tap roots extend no further than 30-45cm below the soil surface. Four to eight axial roots may be encountered which often originate horizontally but point downward. The lateral roots can extend 2m from the trunk. About 80-90% of the feeder root is on the first 20cm of soil and is 60-90cm away from the trunk of the tree (Mavolta, 195-196). However, Nutman states that the greatest root concentration is in the 30-60cm depth. The root systems are heavily affected by the type of soil and the mineral content of the soil. To be thick and strong the root system needs an extensive supply of nitrogen, calcium and magnesium. During planting the main vertical roots are often clipped to promote growth of the horizontal roots, which then have better access to water and added nutrients in the top soil.
The elliptical leaves of the coffee tree are shiny, dark green, and waxy. The leaf area index is between 7 and 8 for a high-yielding coffee (Malavolta, 195). The coffee plant has become a major source of oxygen in much of the world. Each hectare of coffee produces 86lbs of oxygen per day, which is about half the production of the same area in a rain forest (source: Anacafe).