Coffee leaves grow in opposite decussate manner on the main stem and orthotropic shoots (growing along the vertical axis).
But, in plagiotropic branches (the longer axis inclined away from the vertical axis), the leaf orientation is opposite due to torsion. The leaves are shiny, wavy and dark green in colour with conspicuous veins. The shape of the leaf is usually elliptical. The leaves of Arabica are slender and more delicate than those of Robusta or Liberica. The leaf tip varies from pointed (acuminate) to obtuse. As mentioned earlier, the young leaves of Arabica are either light green or bronze while in Robusta and Liberica, the young leaf colour is generally dark bronze.
A silent feature of coffee leaves, as in the case of some other members of Rubiaceae, is the occurrence of ‘domatia’ which are small opening on the lower surface of leaves in the angles of the veins intersecting the mid-rib. They do not appear to have any specific function. In coffee, stomata (minute opening on the leaf for gas exchange) are present only on the lower surface of the leaves and the number of stomata vary from species to species (10000 to 17500cm2). The stomatal level number is negatively correlated to ploidy level with Arabica showing less number of stomata compared to Robusta.
Coffee is a short day plant i.e., floral initiation takes place during short place during short day conditions of 8 to 11 if day light which is prevalent between September to December in South India.
Flower buds are produced at the axis of mature green wood on short stalks which are known as peduncles. The group of flowers, technically called ‘inflorescence’ is a condensed cymose (a type of inflorescence where the terminal bud is formed and further flower formation is through lateral buds, as opposed to racemose inflorescence) subtended by bracts (modified vegetative structure at the base of inflorescence which is protective in function). In Robusta, bracts are leafy and expanded whereas they are small and scaly in Arabica,4 to 5 inflorescence of 1 to 4 flowers each other produced per axil while in Robusta more number of flowers per inflorescence(5 to 6) is commonly produced.
In Arabica, the axillary buds are indeterminate i.e., they may produce either vegetative shoots or flower buds depending upon the seasonal factors like temperature, moisture and photoperiod. In Robusta floral differentiation is faster than in Arabica and also appears to be determinate. The flower buds grow to a length of 7 to 8 mm after initiation and then remain quiescent until stimulated into flowering. Rain or irrigation after a dry period includes further growth in flower buds which blooms within 8 to 10 days.
Under adverse conditions, particularly at high temperatures, abnormal flowers called ‘star-flowers’ may occur in some plants. This phenomenon is under genetic control but is also influenced by environmental factors such as high temperatures, radiation, prolonged dry weather and inadequate blossom showers. The petals of these flowers are small, fleshy and often green in colour. The male and female parts are rudimentary. They may remain in this condition till they receive water and grow into partially successful blossom nut the fruit set would be poor.
In Robusta, flowering normally occurs on 8th day.in Arabica, flowering occur in phased manner on 8th, 9th and 10th day (20%,50% and 30%), which however depends on day and night temperature and also the quantum of rain /water received.