growing cells as a result. this directional growth can occur via a plant's response to a particular stimulus, such as light (phototropism), gravity (gravitropism), water, (hydrotropism), and physical contact (thigmotropism). plant growth and development are mediated by specific plant hormones and plant growth regulators (pgrs) (ross et al. 1983). endogenous hormone levels are influenced by plant age, cold hardiness, dormancy, and other metabolic conditions; photoperiod, drought, temperature, and other external environmental conditions; and exogenous sources of pgrs, e.g., externally applied and of rhizospheric origin. morphological variation plants exhibit natural variation in their form and structure. while all organisms vary from individual to individual, plants exhibit an additional type of variation. within a single individual, parts are repeated which may differ in form and structure from other similar parts. this variation is most easily seen in the leaves of a plant, though other organs such as stems and flowers may show similar variation. there are three primary causes of this variation: positional effects, environmental effects, and juvenility. evolution of plant morphology transcription factors and transcriptional regulatory networks play key roles in plant morphogenesis and their evolution. during plant landing, many novel transcription factor families emerged and are preferentially wired into the networks of multicellular development, reproduction, and organ development, contributing to more complex morphogenesis of land plants. most land plants share a common ancestor, multicellular algae. an example of the evolution of plant morphology is seen in charophytes. studies have shown that charophytes have traits that are homologous to land plants. there are two main theories of the evolution of plant morphology, these theories are the homologous theory and the antithetic theory. the commonly accepted theory for the evolution of plant morphology is the antithetic theory. the antithetic theory states that the multiple mitotic divisions that take place before meiosis, cause the development of the sporophyte. then the sporophyte will development as an independent organism. developmental model organisms much of developmental biology research in recent decades has focused on the use of a small number of model organisms. it has turned out that there is much conservation of developmental mechanisms across the animal kingdom. in early development different vertebrate species all use essentially the same inductive signals and the same genes encoding regional identity. even invertebrates use a similar repertoire of signals and genes although the body parts formed are significantly different. model organisms each have some particular experimental advantages which have enabled them to become popular among researchers. in one sense they are "models" for the whole animal kingdom, and in another sense they are "models" for human development, which is difficult to study directly for both ethical and practical reasons. model organisms have been most useful for elucidating the broad nature of developmental mechanisms. the more detail is sought, the more they differ from each other and from humans. plants thale cress (arabidopsis thaliana) vertebrates frog: xenopus (x. laevis and x. tropicalis). good embryo supply. especially suitable for microsurgery. zebrafish: danio rerio. good embryo supply. well developed genetics. chicken: gallus gallus. early stages similar to mammal, but microsurgery easier. low cost. mouse: mus musculus. a mammal with well developed genetics. invertebrates fruit fly: drosophila melanogaster. good embryo supply. well developed genetics. nematode: caenorhabditis elegans. good embryo supply. well developed genetics. low cost. unicellular algae: chlamydomonas yeast: saccharomyces others also popular for some purposes have been sea urchins and ascidians. for studies of regeneration urodele amphibians such as the axolotl ambystoma mexicanum are used, and also planarian worms such as schmidtea mediterranea. organoids have also been demonstrated as an efficient model for development. plant development has focused on the thale cress arabidopsis thaliana as a model organism....