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5 Types of Culture A plant consists of different organs and each organ is composed of different tissues, which in turn are made up of different individual cells. Plant tissue culture refers to the in vitro cultivation of plants, seeds, plant parts (tissues, organs, embryos, single cells, protoplasts) on nutrient media under aseptic conditions. The procedures of plant tissue culture have developed to such a level that any plant species can be regenerated in vitro through several methodologies. Unlike animal cells, plant cells, even highly mature and differentiated, retain the ability to change to a meristematic state and differentiate in to a whole plant if it has retained an intact membrane system and a viable nucleus. In plant tissue culture, more often we use an explant (an excised piece of differentiated tissue or organ) to initiate their growth in culture. The non-dividing, differentiated, quiescent cells of the explant when grown on a nutrient medium first undergo changes to achieve the meristematic state. The phenomenon of mature cells reverting to a meristematic state and forming undifferentiated callus tissue is termed as dedifferentiation. Since the multicellular explant comprises cells of diverse types, the callus derived will be heterogeneous. The ability of the component cells of the callus to differentiate into a whole plant or a plant organ is termed as redifferentiation. These two phenomena of dedifferentiation and tedifferentiation are inherent in the capacity of a plant cell, and thus giving rise to a whole plant is described as cellular totipotency. Generally, a callus phase is involved before the cells can undergo redifferentiation leading to the regeneration of a whole plant. The dedifferentiated cells can rarely give rise to whole plants directly without an intermediate callus phase. CYTODIFFERENTIATION Cell differentiation is the basic event of development in higher organisms. The cells ina callus are parenchymatous in nature; the differentiation of these cells into a variety of cells is required during redifferentiation of cells into whole plants which is conveniently referred to as cytodifferentiation. In vitro and in vivo, the main emphasis in plant cytodifferentiation has been laid on vascular tissue differentiation (xylem, phloem), particularly the xylem elements. In an intact plant, tissue differentiation goes on in a fixed manner that is characteristic of the species and the organ. Callus cultures, which lack vascular elements offer a valuable system for the study of the effect of various factors that control cellular totipotency through cytological, histological and organogenic differentiation. 
