Leaves are lateral outgrowth of a plant shoot which are initiated by the shoot meristem. They have finite growth, are vascularized and usually photosynthetic.
After zygote formation, the embryo is formed inside the seed which leads to embryonic leaves and shoot apical meristem after germination. The embryonic leaves are known as cotyledons. In post-embryonic development, these embryonic leaves give rise to true leaves. The apical cell divides transversely to form upper tier and lower tier. Further transverse division in lower tier gives rise to upper lower tier and lower tier. The upper tier forms the apical tissues- the shoot meristem and cotyledons. The upper lower tier forms the base of cotyledons. The lower tier forms the hypocotyl and most of the root. The root tip including the meristem is derived from hypophysis.
Phyllotaxis or phyllotaxy is the arrangement of leaves on a plants stem. The basic phyllotactic patterns in the plant kingdom are either opposite, whorled or alternate.
Types of phyllotaxis
Opposite phyllotactic pattern In this leaf primordia grow one by one. This type of pattern can be further divided into two; distichous or decussated phyllotaxis. In distichous phyllotaxis, as in maize, successive leaf primordia are placed each at 180 degrees from the previous one, in decussated phyllotaxis as in Coleus species, successive leaf pairs are perpendicular.
Whorled phyllotactic pattern In case of whorled phyllotaxis, at least three leaf primordia grow at the same node on the stem. Primordia in a node are evenly spread around the stem, mid-way between those in the previous node.
Alternate phyllotactic pattern More than 80% of the higher plant species have an alternate phyllotaxis, as in the case of potato, araucaria, yucca or sunflowers. Alternate phyllotaxis can be further divided into spiral or multijugate phyllotaxis.
In spiral phyllotaxis, leaf primordia grow one per node, each at a constant divergence angle of degrees from the previous.
In multijugate phyllotaxis, two or more leaf primordia grow at the same node. Leaf primordia of a node are spread evenly around the stem, each group of leaf primordia of a node is at a constant divergence angle of 137.5 degrees from the leaf primordia group of the previous node.
The phyllotactic form has resulted in the proliferation of terminology like bipinnate, orthodistichy, spirodistichy, spirodecussation and many more. The simplest type of phyllotaxis is called “alternate”. In this phyllotaxis, the primordia develop one at a time on opposite sides of the apex. The resultant structure is almost planar with the organs alternating in the direction along the stem. A closely related type of phyllotaxis is called ―spiral‖. In the apical ring, the primordia develop one at a time. Each primordium forms at a fixed angle from the previous primordium, this is known as the divergence angle. As the plant grows the plant organs tend to form a spiral helix around the stem. The divergence angle is 180º in spiral phyllotaxis. Nodes are the places where the plant organs are attached to the plant. When the nodes are equally spaced from each other primodia formation takes place. When more than one primordia forms at a time the plant is said to form a ―whorld. Dimerous is when whorls composed of two primordial. With three primordia are called ―trimerous and so on. The whorl present primordia are often assumed to be equally space along the apical ring. When successive layers of dimerous whorls form at 90o with respect to each other we get what is known as “opposite” or “decussate” phyllotaxis. The multijugate phyllotaxis occurs when successive layers of whorls do not form exactly halfway between the previous layers. Multijugate phyllotaxis along with dimerous is known as “bijugate”, and with trimerous whorls it is called ―trijugate and so on. It can be said that multicussate phyllotaxis is a special case of multijugate phyllotaxis. By lowering the temperature, the developmental process is slowed down, causing primordial that otherwise would have formed simultaneously in a whorl to form one at a time. It shows that whorled phyllotaxis and spiral phyllotaxis are closely related. All of these forms that are discussed are present in primitive plants. Sometimes one type of phyllotaxis is exhibited in the juvenile stage of a plant while another type is exhibited in the adult stage, e.g. eucalyptus.
Transition to flowering
The transition to flowering in plants requires the reprogramming of the shoot apical meristem. The vegetative meristem is thought to first pass through a juvenile phase. During this phase, the vegetative meristem is not able to respond to any signal, internal or external that would trigger flowering. The attainment of reproductive competence is often marked by changes in the morphology or physiology of vegetative structures. In some species, environmental factors influence the timing of flowering; it shows the time of year and growth conditions favorable for sexual reproduction and seed maturation. These factors include photoperiod (i.e., day length), light quality (spectral composition), light quantity (photon flux density), vernalization (exposure to a long period of cold) and nutrient and water availability. Some species are less sensitive to environmental variables and appear to flower in response to internal cues such as plant size or number of vegetative nodes. Flowering can also be induced by stresses such as nutrient deficiency, drought and overcrowding. This response enables the plant to produce seeds, which are much more likely to survive the stress than is the plant itself. Over the years, physiological studies have led to three models for the control of flowering time.
The florigen concept
It was based on the transmissibility of substances or signals across grafts between reproductive donor shoots and vegetative ―recipients‖. It was proposed that florigen, a flower-promoting hormone, was produced in leaves under favorable photoperiods and transported to the shoot apex in the phloem. The identification of a graft-transmissible floral inhibitor also led to the concept of a competing ―antiflorigen‖. Many research years were consumed hunting for florigen in the phloem sap, but its chemical nature has remained elusive. The inability to separate the hypothetical flowering hormones from assimilates led to a second model.
The nutrient diversion hypothesis
This model proposed that inductive treatments result in an increase in the amount of assimilates moving to the apical meristem, which in turn induces flowering. The view that assimilates is the only important component in directing the transition to flowering was superseded by the multifactorial control model.
Multifactorial control model
It proposes that a number of promoters and inhibitors, including phytohormones and assimilates, are involved in controlling the developmental transition. According to this model, flowering can only occur when the limiting factors are present at the apex in the appropriate concentrations and at the right times. This model attempted to account for the diversity of flowering responses by proposing that different factors could be limiting for flowering in different genetic backgrounds and/or under particular environmental conditions.