The origin and development of plant form and structure. Morphogenesis may be concerned with the whole plant, with a plant part, or with the subcomponents of a structure.
The establishment of differences at the two ends of a structure is called polarity. In plants, polar differences can be recognized very early in development. In the zygote, cytological differences at the two ends of the cell establish the position of the first cell division, and thus the fate of structures produced from the two newly formed cells. During the development of a plant, polarity is also exhibited in the plant axis (in the shoot and root tips). If a portion of a shoot or root is excised and allowed to regenerate, the end toward the shoot tip always regenerates shoots whereas the opposite end forms roots. Polarity is also evident on the two sides of a plant organ, such as the upper and lower surface of a leaf, sepal, or petal.
The diversity in plant form is produced mainly because different parts of the plant grow at different rates. Furthermore, the growth of an individual structure is different in various dimensions. Thus the rate of cell division and cell elongation as well as the orientation of the plane of division and of the axis of cell elongation ultimately establish the form of a structure. Such differential growth rates are very well orchestrated by genetic factors. Although the absolute growth rates of various parts of a plant may be different, their relative growth rates, or the ratio of their growth rates, are always constant. This phenomenon is called allometry (or heterogony), and it supports the concept that there is an interrelationship between the growth of various organs of a plant body. See Plant growth
During development, either the removal of or changes in one part of the plant may drastically affect the morphogenesis of one or more other parts of the plant. This phenomenon is called correlation and is mediated primarily through chemical substances, such as nutrients and hormones.
The ultimate factors controlling the form of a plant and its various organs are the genes. In general, several genes interact during the development of a structure, although each gene plays a significant role. Thus, a mutation in a single gene may affect the shape or size of a leaf, flower, or fruit, or the color of flower petals, or the type of hairs produced on stems and leaves. There are at least two classes of genes involved in plant morphogenesis: regulatory genes that control the activity of other genes, and effector genes that are directly involved in a developmental process. The effector genes may affect morphogenesis through a network of processes, including the synthesis and activity of proteins and enzymes, the metabolism of plant growth substances, changes in the cytoskeleton and the rates and planes of cell division, and cell enlargement. See Gene action, Plant hormones
Plant form is also known to be affected by nutritional factors, such as sugars or nitrogen levels. For example, leaf shape can be affected by different concentrations of sucrose, and the sexuality of flowers is related to the nitrogen levels in the soil in some species. Inorganic ions (such as silver and cobalt) have also been known to affect the type of flower produced. See Plant mineral nutrition
Although genes are the ultimate controlling factors, they do not act alone, but interact with the existing environmental factors during plant development. Environmental factors, including light, temperature, moisture, and pressure, affect plant form. See Physiological ecology (plant), Plant-water relations