Once aerial roots reach the substrate, lateral roots develop, with a very different structure from that of aerial ones: trichosclereids are not present, chlorophyll is absent, and the differentiation of protoxylem begins after the short elongation zone (Gill and Tomlinson, 1977). These roots also contained chlorophyll, trichosclereids (long fiber cells with lignified walls), which are more typically found in stems (Gill and Tomlinson, 1971). Considerable anatomical differences in aerial roots from belowground ones have been observed in mangroves in the family Rhizophoraceae, in which cell divisions in aerial roots were observed over a length of up to 23 cm. Although relatively less is known about aerial root growth, there are some studies on the anatomy and morphology of aerial roots in general and aerial roots of epiphytes in particular. It has been claimed for some species that the growth zone of aerial roots can reach many tens of centimeters (Jost, 1908), which suggests a different organization of growth processes from subterranean roots. Even at the dawn of experimental biology, aerial root growth was observed to be different from that of the substrate roots (Went, 1895 Linsbauer, 1907 Blaauw, 1912). We hypothesized a fundamentally different structure and nature of growth in aerial roots because these do not experience substrate root-soil resistance (Eskov et al., 2016). In some species, the meristem decreases in size and changes from a closed type to an open type during ontogenesis (Armstrong and Heimsch, 1976 Clowes and Wadekar, 1989) or disappears over time. The open type is probably ancestral among angiosperms, but the closed type is most common in extant species (Heimsch and Seago, 2008). Based on cell proliferation direction, there is a closed RAM or an open RAM (Clowes, 1981, 1982 Groot et al., 2004 Evert, 2006), reflecting the presence or absence of a clear anatomical boundary between root cap and root proper. The root apical meristem (RAM) is located in the growing tip of the main, subordinate, or lateral root. Currently, the largest study on root growth compared 73 flowering plant species, including only ruderal geophyte seedlings and crops (Zhukovskaya et al., 2018). As a rule, pertinent studies utilized a rhizotron and on agricultural crops juvenile plants (Cahn et al., 1989). Nowadays, the distribution of growth in the root is considered as a most effective system for study than growth in other plant organs, and the root is characterized by strictly columnar growth of all cells in the meristem (Evert, 2006).ĭata on root growth rates are relatively scarce in the literature. The Sachs descriptions are applicable to some cells and organs, but not for all of them. The apical meristem of such roots is characterized by active proliferation, laying of cell rows with symplastic growth, and rapid change in cellular organization. The vast majority of cellular root growth studies have been performed on a limited number of species, mainly the roots of crop seedlings and Arabidopsis thaliana, which have long been the primary models for the study of the root apical meristem (RAM), its organization, ontogenesis, and growth regulation mechanisms. Sachs ( 1887), by direct observation, was apparently the first who have described three stages of plant organ growth: (1) meristematic, during which new cells are formed and separated into tissues (2) elongation, with rapid cell growth until organs reach their final size and (3) maturation, when final features of different cell types develop.
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