LIFE CYCLE
The life cycle of .

plants may belong to any one of a number of varieties which follow somewhat different growth patterns. The following outline describes the more common form of growth. Differences between varieties can be thought of as variations on this standard theme.

is an annual plant. A single season completes a generation, leaving all hope for the future to the seeds. The normal life cycle follows the general pattern described below.

Seed Germination

With winter past, the moisture and warmth of spring stir activity in the embryo. Water is absorbed and the embryo's tissues swell and grow, splitting the seed along its suture. The radical or embryonic root appears first. Once clear of the seed, the root directs growth downward in response to gravity.
 

Meanwhile, the seed is being lifted upward by growing cells which form the seedling's stem. Now anchored by the roots, and receiving water and nutrients, the embryonic leaves (cotyledons) unfold. They are a pair of small, somewhat oval, simple leaves, now green with chlorophyll to absorb the life-giving light. Germination is complete. The embryo has been reborn and is now a seedling living on the food it produces through photosynthesis. The process of germination is usually completed in three to 10 days.

Seedlings

The second pair of leaves begins the seedling stage. They are set opposite each other and usually have a single blade. They differ from the embryonic leaves by their larger size, spearhead shape, and serrated margins. With the next pair of leaves that appears, usually each leaf has three blades and is larger still. A basic pattern has been set. Each new set of leaves will be larger, with a higher number of blades per leaf until, depending on variety, they reach their maximum number, often nine or 11. The seedling stage is completed within four to six weeks.

Vegetative Growth

This is the life cycle period of maximum growth. The plant can grow no faster than the rate that its leaves can produce energy for new growth. Each day more leaf tissue is created, increasing the overall capacity for growth. With excellent growing conditions, has been known to grow six inches a day, although the rate is more commonly one to two inches. The number of blades on each leaf begins to decline during the middle of the vegetative stage. Then the arrangement of the leaves on the stem (phyllotaxy) changes from the usual opposite to alternate. The internodes (stem space from one pair of leaves to the next, which had been increasing in length) begin to decrease, and the growth appears to be thicker. Branches which appeared in the axils of each set of leaves grow and shape the plant to its characteristic form. The vegetative stage is usually completed in the third to fifth months of growth.

 

LIFE CYCLE PHOTOPERIOD AND FLOWERING
For the grower the most important plant/environment interaction to understand is the influence of the photoperiod. The photoperiod is the daily number of hours of day (light) vs. night (dark). In nature, long nights signal the plant that winter is coming and that it is time to flowers and produce seeds. As long as the day-length is long, the plants continue vegetative growth. If female flowers do appear, there will only be a few. These flowers will not form the characteristic large clusters or buds. If the days are too short, the plants flowers too soon, and remain small and underdeveloped.

The plant "senses" the longer nights by a direct interaction with light. A flowering hormone is present during all stages of growth. This hormone is sensitive to light and is rendered inactive by even low levels of light. When the dark periods are long enough, the hormones increase to a critical level that triggers the reproductive cycle. Vegetative growth ends and flowering.

The natural photoperiod changes with the passing of seasons. In the Northern Hemisphere, the length of daylight is longest on June 21. Day-length gradually decreases until it reaches its shortest duration on December 22. The duration of daylight then begins to increase until the cycle is completed the following June 21. Because the Earth is tilted on its axis to the sun, day-length also depends on position (or latitude) on Earth. As one moves closer to the equator, changes in the photoperiod are less drastic over the course of a year. At the equator (0 degrees altitude) day length lasts about 12.5 hours on June 21 and 11.5 hours on December 22. In Maine (about 45 degrees north), day-length varies between about 16 and nine hours. Near the Arctic Circe on June 21 there is no night. On December 22 the whole day is dark. The longer day-length toward the north prevents from flowering until later in the season. Over most of the northern half of the country, flowering is often so late that development cannot be completed before the onset of cold weather and heavy frosts.

The actual length of day largely depends on local conditions, such as cloud cover, altitude, and terrain. On a flat Midwest plain, the effective length of day is about 30 minutes longer than sunrise to sunset. In practical terms, it is little help to calculate the photoperiod, but it is important to realise how it affects the plants and how you can use it to your advantage.

life cycle generally needs about two weeks of successive long nights before the first flowers appear. The photoperiod necessary for flowering will vary slight with (1) the variety, (2) the age of the plant, (3) its sex, and (4) growing conditions.

 

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Life Cycle