A computational model of the effect of symplastic growth on cell mechanics in a linear leaf blade

The epidermis of a linear leaf, as in Poaceae, is established by parallel files of cells originating from the leaf base. Their feature is symplastic growth where neighboring cell walls adhere and do not slide along each other. We developed a simple mechanical cell-based model for symplastic growth of linear leaf blade. The challenge is to determine what restrictions on cell size symplastic growth creates compared to the free growing cells. We assume an unidirectional growing cell ensemble starting from a meristem-like layer of generative cells and then generating parallel cell rows from every cell of the initial layer. Each cell is characterized by its growth function, and growth of the whole leaf blade is accompanied by mutual adjustment between all the cells. Cells divide once they have reached a threshold area. A mathematical model and its implementation are proposed for computational simulation of 1D symplastic growth of tissues. The question analyzed is how a cell grows in a plant tissue if there is a mechanism for regulating the growth of an isolated growing cell and the behavior of the cell wall matter is elastoplastic. The results of the simulation of linear leaf blade growth are compared to those for a free-growing cell population.

Tissues development in stems of Aristolochia clematitis L. in the point of view of multicellular complexes formation

After cytokinesis the cells do not separate but remain within the wall of the mother cell. After a series of divisions a multicellular complex arises. In the stems of <em>Aristolochia clematitis</em> procambium is closer related to protoxylem than to protophloem, and metaphloem is closer related to metaxylem than to protophloem. Since protophloem has a closer common origin with fibre primordia than with the remaining tissues, it cannot be decided unequivocally what is the origin of the fibres or when procambium differentiates. The common origin of the primary vascular tissues is visible in the pattern of the multicellular complexes, whereas the common origin of the secondary vascular tissue developing in the underground several-year-old parts of the stem can be traced in the arrangement of the single radial tiers. Some characteristics of symplastic growth are discussed.

Position of rays and lateral deviation of vessel elements in the stem wood of some dicotyledonous species with storeyed, double-storeyed, and nonstoreyed cambia

It is believed that differentiating vessel elements increase their diameter by growing intrusively in the circumferential direction and symplastically in the radial direction in relation to the stem axis. On the basis of a detailed analysis of the cell arrangement observed in a series of semithin anatomical sections of cambial zone and the developing and mature secondary xylem of Terminalia ivorensis , Wisteria floribunda , and Millettia laurentii , we revealed a novel correlation of growing vessel elements with surrounding tissues. Rays seem to prevent the growing vessel elements from protruding laterally between the cells of adjacent rays. The growing vessel elements break the continuity of several neighbouring radial files of fusiform cell derivatives but not of ray cell derivatives. If a contiguous ray becomes an obstacle for the growth of vessel elements on any one side, the growing elements often start to grow in the opposite direction, consequently causing a deviation in the alignment of the vessel elements concerned. This mechanism explains why vessel elements may deviate from the array of their precursors, the fusiform cambial initials. Our models on the intrusive symplastic growth of vessel element mother cells have revealed that intrusive growth does not occur between radial walls of neighbouring cells.

Does intrusive growth of fusiform initials really contribute to circumferential growth of vascular cambium?

Currently it is believed that intrusive growth of fusiform cambial initials adds to the circumference of the cambial cylinder: the initial cells multiplied by anticlinal divisions are produced in excess, and the excess cells are later eliminated from the cambial surface. The present study, dealing with the intrusive growth of fusiform initials in the cambium of Laburnum anagyroides Medik, suggests that addition of a radial file of initials owing to intrusive growth, or elimination of any such file, has no visible effect on the tangential dimensions of a given cambial sector, and that intrusive growth of fusiform initials and the elimination of excess initials occur in unison. The two events complement each other, and the gain in size of the growing fusiform cell is accompanied by a reduction in size of its neighbour cell, thus keeping the tangential dimensions of the cambium unchanged. Our findings on L. anagyroides find support from illustrations of previous studies, which we have re-examined and re-interpreted. Our data suggest that increase in the cambial circumference is largely due to the symplastic growth of the fusiform initials in tangential direction.

Apical intrusive growth of cambial fusiform initials along the tangential walls of adjacent fusiform initials: evidence for a new concept

A new study of cambium of Pinus sylvestris L., Tilia cordata Mill. and Wisteria floribunda (Willd.) DC provides fresh clues on the cambial dynamics, rejecting the hitherto held concept that intrusive growth of the fusiform initial occurs between the radial walls of adjacent initials. It demonstrates that intrusion of the elongating initial in fact takes place along tangential walls of adjacent fusiform initials and their immediate derivatives. It also suggests a new mechanism for ‘elimination of initials’. Intrusive growth of the fusiform initial was found to begin with development of characteristic slants, representing a transitional stage of the process of transformation of periclinal walls of fusiform initial cells into radial walls, as observed in transverse sections of active cambium. The gradually progressing event comprised (a) appearance of either a triangular microspace limited by two periclinal walls of a fusiform initial and its derivative and one radial wall of another fusiform initial in the adjacent radial file, or a rhomboidal microspace enclosed by four periclinal walls of two laterally adjacent fusiform initials and their immediate derivatives, (b) intrusion of elongating tip of fusiform initial from neighbouring file into the microspace thus formed, (c) symplastic growth of the cambial cell walls in radial direction, (d) unequal periclinal divisions of fusiform initial cells while growing intrusively, and (e) unequal periclinal divisions of derivative cells not growing intrusively. Intrusive growth between periclinal walls affected rearrangement of the fusiform initials but did not add to the cambial circumference. The existing concepts of (a) intrusion of the fusiform initial between radial walls of neighbouring initials and (b) elimination of fusiform initials from cambial surface have been reassessed and redefined.

THE ELONGATION OF FUSIFORM CAMBIAL CELLS IN CHAMAECYPARIS

Studies of the secondary xylem and phloem indicated that after their origin in anticlinal division, sister fusiform initials usually elongated rapidly, particularly at their overlapping tips. After this first phase, rate and amount of extension varied, both along the overlap and at the opposite ends. Often elongation proceeded in a somewhat periodic fashion, being determined in part by inherent factors and in part by position and behavior of the adjoining cells. Sometimes elongating tips were temporarily stalled at blocking rays, while at other times very rapid extension occurred in association with loss of a neighboring initial from the cambium. Growth appeared to be apical, "intrusive" in the sense that the elongating tips thrust between other cells. No evidence was found to support the theory of simultaneous elongation of considerable portions of adjoining walls by "symplastic" growth.