The radicle root will emerge two to three days after germination. The radicle will become the taproot and grows down in the soil. The taproot can grow up to 10 inches or more when the cotyledons unfold at day five to seven.
Cotyledons, or seed leaves, are the first visible structures above the ground. They serve as an energy storage system and provide nutrition to the plant until the first true leaf forms. Cotyledons will be opposite of each other on the stem and will be the only leaf structures that originate in this pattern on the cotton plant. The first true leaf is produced about a week after the cotyledons appear. After the first leaf has emerged, the apical meristem moves from a storage unit to facilitating photosynthesis. During this early stage of development, cotton growth is referred to by the number of leaves, 2 leaf, 3 leaf, etc.
Cotton plants develop two different types of branches, vegetative and fruiting. The vegetative branches are straight growing branches for producing energy through photosynthesis. Fruiting branches grow in a zigzag pattern and contain multiple meristems and form fruiting buds. The first fruiting branch will form around node five or six. New fruiting branches develop around every three days and squares form at new positions on fruiting branches every six days. At each node, both a vegetative and fruiting branch will develop. These branches develop up the main stem in a spiral pattern in a three-eighths phyllotaxy above the cotyledons. At each fruiting position along a fruiting branch, a new leaf will also develop to feed the developing fruit.
Reproductive growth can be characterized by square (fruiting bud) development and node location on the plant. The square development is a pre-bloom flower bud that forms at the initiation of a fruiting branch. The first square position is the location where the first square is produced. As the squares develop, the branch between the main stem and the square grows and an axillary meristem develops on the branch adjacent to the square. The axillary meristem will produce a second position square and a subtending leaf. The first square typically forms on nodes five to six. During this stage, cotton is characterized by the size of the square. In the figure below, squares are defined as pinhead square, matchhead square, and candle square. The pinhead square is the first stage that can be identified during bud development. The match-head square is the next stage, this is when the bud is about one-third grown. The next stage is the candle, this is just prior to the bud blooming.
20-25 days after squaring begins, first bloom will occur. When a bloom opens, it will be a white bloom and can be pollinated within a few hours. Blooms will turn pink on the second day, and red by the third day. If the bloom was pollinated, a young boll will develop under the bloom. If pollination was unsuccessful, the plant will abort that position. During the bloom stage, cotton is often referred to by the number of weeks it has been blooming, second week of bloom, etc.
Bolls require about 50-60 days to open. The fibers are sensitive to environmental conditions. There are three phases to boll development: enlargement, filling, and maturation. Enlargement occurs when the fibers are produced. The seeds begin to elongate, and the maximum volume of the boll and seeds are set. The fibers during this stage are a thin-walled tubular structure. This phase typically lasts three weeks. The filling phase begins during the fourth week after flowering, filling is a process known as deposition, where the secondary walls of the cotton fiber form. This phase lasts for about two weeks. Finally, the maturation phase begins when the boll reaches full size and maximum weight. The fibers and seeds mature within the boll. The boll will dry and split, causing the boll to open.
Defoliation is the process of removing the leaves from a cotton plant. With no intervention, a cotton plant will naturally defoliate in the fall, much as deciduous trees. However, to aid in timely harvest and lint quality, cotton in the United States is chemically defoliated. This process begins when the last harvestable boll has reached physiological maturity. Usually, plants are ready for harvest about 2-3 weeks after a defoliation application.