Morphology and Anatomy of Vascular Plants

Abstract

Although every cell in a multicellular organism is an individual living unit, it is not an organism. The organism’s macroscopic structure is as decoupled from its cellular structure as the architecture of a building is from bricks and other construction elements. One could (and indeed did for quite a long time) quite usefully study morphology without ever knowing anything about cells. Complex structures are possible without cellular arrangement (Figs. 4.1, 10.90d, 10.113; see Sect. 2.2.3.6). Of course, the rarity of true large cellular plants and the enormous diversity of multicellular plants show that multicellularity is a more favorable basis for the evolution of large organisms than the enlargement and increased complexity of single cells. Prerequisites for the formation of multicellular organisms include not only the massing together (agglomeration) of cells but also the ordered differentiation of initially identical cells. The differentiation and the functional specialization of somatic cells (Greek: sóma, body) is due to differential gene activation (see Sects. 2.2.3, 6.2.2.3). The signals for the appropriate local differentiation must be conveyed by every cell in a meristem. The multicellular organism can only be formed when all the cells and intercellular signals interact correctly (see Sect. 6.4). The single cell is no longer a biological unit, instead, the supracellular function-sharing of the multicellular vegetative body, the blastema (Greek: blástema, offspring, also shoot or bud). The holistic system is what differentiates the blastema (multicellular organism) from a mere cell colony (coenobium).