Why are meristematic cells small

Vascular plants (cormophytes and some thallophytes) produce new cells throughout their life, and new organs are created at regular intervals. In an embryo all cells are still actively dividing, but as soon as it has exceeded a certain size and has reached a certain stage of differentiation, the production of new cells is usually limited to certain areas (meristems).

Typical meristems occur in the tip regions of all shoots and roots as well as on the main and lateral axes. They are called vertex or apical meristems, or as vegetation points. The shoot meristems are usually protected by bracts and the entire complex forms a bud.

Scanning electron micrograph: Longitudinal section through a point of vegetation (flower bud of the snapdragon (Antirrhinum majus))

from "Snapdragon" © K. STÜBER

Scanning electron micrograph: Longitudinal section through a ripening flower bud of the snapdragon (Antirrhinum majus)

from "Snapdragon" © K. STÜBER

From the existence of the apical meristems, the tip growth is derived as one of the most striking characteristics of plant growth. This, and with it the development of plants, is in stark contrast to the processes involved in animal development. The growth of animals is an allometric process, i.e. all parts of the body always grow equally, so that the proportions to one another are preserved. Once an animal reaches its full size, growth stagnates. Nevertheless, cell divisions take place continuously even in an adult animal. Just think of the cells of the epithelia or of blood-forming cells. However, they only replace used or damaged cells. Repairs of this kind do not occur on plants. For example, if a leaf, flower or whatever is removed or damaged, the affected part is neither replaced nor repaired. Rather, new, often similar parts are created elsewhere - and that in turn does not occur with animals.

Illustration of vegetation cones of Elodea


The cells of a meristem are potentially unrestrictedly capable of dividing, but this does not mean that they are constantly dividing. The principle of apical dominance is known in many plants, which means that meristems in side shoots (= subapical meristems, side buds) are suppressed in their activity as long as the main shoot is growing. This is also an indication of a flow of information between the individual meristems. If the tip of the main shoot is removed, there is no inhibiting effect and the side buds sprout.

In addition to the apical meristems, many plants (gymnosperms, dicotyledons) have extensive lateral meristems (cambium, cork cambium), which cause secondary growth in the thickness of the stem axis. Their activities can fluctuate with the seasons and are reflected, for example, in the appearance of annual rings.

Other plants, e.g. many monocotyledons (grasses, etc.) have intercalary meristems. These are actively growing meristems, clearly separated from the apical meristem, which are inserted between more or less differentiated tissue regions, usually at the base of an internode. The formation of secondary meristems (subsequent meristems) illustrates that cells that have assumed a certain degree of specialization and thus largely restricted the division activity under normal circumstances, give up their differentiation status and dedifferentiate. The willingness to divide is not lost; the cells can be returned to the quasi-meristematic state as soon as external circumstances require it. The widespread use of cuttings in horticulture or, for example, the formation of secondary leaves and roots at the interfaces of a begonia leaf can be used as evidence for this.

For us the question now arises of the exact definition and delimitation of a meristem, and this is where the difficulties begin. First a very simple example:

Dictyota dichotoma (Fork wrack, a brown alga) forms a flat thallus, the thickness of which comprises three cell layers. A large apical cell sits apically, which usually divides periclinically. The daughter cell secreted downwards (subapical, basalward) divides anticlinically, in the daughter cells formed subsequently periclinic and anticlinic divisions take place alternately. The thallus thus increases both in length and in width. The crown cell also divides anticlinically at regular intervals. As a result, two equivalent vertex cells develop, the divisions of which in turn lead to a fork in the thallus. As simply as the scheme of the divisions can be explained in this example, it does not define the delimitation of the meristem from the differentiated thallus cells. In the classic SACHS scheme (see illustration on the right) of a cormophyte, this fact is represented by a gradient of the division activity. The meristematic properties are not lost from one division to the next, but only gradually lost.

In contrast to the flat thalli of many algae, the corm is usually a three-dimensional body. In some liverworts, the deciduous mosses and simple vascular cryptogams (horsetail and many ferns) a single tetrahedral apex cell (initial cell) forms the tip of the vegetation point. Daughter cells are released from it in regular alternation of the planes of division in three directions (towards the basal). The daughter cells, in turn, can undergo anticline and pericline divisions

In the phanerogams, the apex is formed by a whole group of cells (initial cells) without any sign of specialization. Instead of the term vegetation point, it is therefore more appropriate to use the term vegetation zone or vegetation cone (examples: Acacia, Oxypolis, Wheat ear). It was also shown that the apical meristem is structured in several layers, with angiosperms (and some gymnosperms) distinguishing between outer tunic layers and the centrally located corpus (A. SCHMIDT, 1924; FAL CLOWES, 1924; A. FÖRSTER, 1943) . In the meristematic cells of the tunic predominantly anticlinic divisions take place, in the cells of the corpus both anticlinic and periclinic divisions take place.

In some sections of the corpus, certain directions of division are predominant, and this is a first step towards tissue differentiation. The epidermis usually develops from the outer layer of the tunic. The remaining (internal) tissues of the plant are derived from cells of the corpus, the tunic, or both.

These schemes are certainly a useful guideline for understanding plant development, because they indicate the appearance of positional information that says that a cell will only develop (in a certain direction) if it is in the "correct" position within of a tissue. This in turn means that cells that have moved into a new position due to external influences (e.g. damage) take on new tasks, i.e. redifferentiate themselves. At the beginning of the 20th century G. HABERLANDT (Universitfit Graz, later University Berlin) established the theorem of the totipotency of plant cells. This says that every cell can develop into a whole plant. However, as the examples given so far show, the principle applies to many, but not to all cell types. We will come back to this elsewhere (protoplasts and tissue cultures). But it follows from everything that growth through cell division, although typical of the meristematic state, is not restricted to the vegetation or growth area. A meristem in no way only includes the initial cells and their immediate derivatives, but includes, as already indicated, sections of the shoot of different lengths.

The situation in root meristems is basically similar, although the anatomical relationships are different. As we have seen, the vegetation point of the shoot is protected by bracts. In the root, the root cap takes on this task. It is constantly regenerated by cells that are continuously secreted to the outside by the meristem. These cells are subject to a high turnover rate ("turn over") because those furthest outward are damaged by the growth-related advancement of the root tip and friction with soil particles and thus used up.

Usually a distinction is made between primary and secondary shoot growth. The former is associated with shoot elongation, the creation of leaves and differentiation of the individual tissue types, the latter with (secondary) growth in the thickness of the shoot axis and the creation of new (secondary) conductive tissue (A. de BARY, 1877). Although we will only deal with the conductive tissue in detail later, it should already be noted that it always consists of the two functional units xylem and phloem. Both are united to form vascular bundles. In sprouts of the seed plants the xylem is on the inside (in the direction of the axis), the phloem on the outside (in the direction of the periphery), in the petioles and leaf veins the xylem is on top, the phloem on the bottom; In plants with secondary growth in thickness (fossil pteridophytes, fossil and recent gymnosperms, some (lignified dicotyledons) they are separated from each other by a meristematic tissue (cambium). In addition, many plant species have another dividing tissue, the phellogen or cork cambium, which is necessary for ongoing production of secondary closure tissue is required.

The procambium is a meristematic tissue that is determined to differentiate into a primary conductive tissue. It arises just below the vegetation point and always where new leaf systems (leaf primordia) appear. In the area of ​​the shoot tip, the development of the vascular tissue is therefore closely linked to the formation of new leaves. Leaves and the conductive tissue connected to them are created in a growth process. The vascular bundles leading into the leaves are called leaf traces. The cells of the procambium are usually united into strands, represent an extension of the vascular bundle into the vegetation point and thus secure the connection of newly formed leaves to the vascular tissue. The procambium cells are stretched, they enlarge (lengthen) in the course of their development. The volume of their vacuoles increases considerably so that they appear more transparent (lighter) than their neighboring cells. This stage is usually seen as the crucial step on the way to differentiation into conductive tissue cells.

Growth processes in the first two millimeters of the tip of the root Allium cepa (Kitchen onion). The following determinations were made to characterize the growth at the indicated levels. Left: number of cells in a 100 µm thick cross-section. Right: cross-sectional area; Length and surface of the cells at different distances from the root tip (after W. A. ​​JENSEN and M. ASHTON, 1960).

In plants with secondary growth, some of the procambium cells remain in a meristematic state and thus become cambium cells. The characteristics of the procambium, which have been briefly outlined so far, show that it represents only a transitional state. It can therefore be present in different stages of development at different heights (levels) of the shoot tip. Now the question arises as to the causes of its origin. Two possibilities are conceivable:

  1. The cause for the formation of the procambium is to be found in cell-cell interactions within the tissue association of the vegetation point, which determine the leaf position pattern and thus the position of the vascular bundle systems.

  2. The induction is based on differentiated tissues. The information about this reaches the vegetation point via already completed procambium cells.

The first assumption could be confirmed experimentally, because the formation and further development of the procambium and the leaf position pattern remain undisturbed in isolated vegetation points. The cells are determined at a point in time at which no morphological changes are still visible. In contrast to the term differentiation, which is used to describe a (mostly irreversible) change in structures and functions in certain specialized directions, determination means the (also mostly irreversible) triggering of these processes.

Long before a visible or measurable change can be recognized, processes take place on the levels of gene expression, metabolism and the regulation of cell activities, as the later consequence of which the visible changes appear.

The cambium is the prototype of a lateral meristem. Depending on its origin, it has one or more layers and usually forms a cohesive layer of cells that is close to the periphery of the shoot (and the root) and takes the form of a hollow cylinder.

Where present, it separates the xylem from the phloem, and as already mentioned, it arises from the procambium in the area of ​​the vascular bundles, which ensures continuity of the meristematic state. In the sections between the vascular bundles, the cambium is formed from parenchymatic (i.e. already differentiated) cells. To take this into account, a distinction is made between the fascicular and the interfascicular cambium (= cambium within vascular bundles and cambium between the vascular bundles). The interfascicular cambium must therefore be viewed as a subsequent meristem or a secondary meristem. A few monocots (dragon tree [Dracaena], yucca, aloe i.a.) have a multi-layered extra-fascicular cambium.

The cells of the cambium are often referred to as initials because after division they initiate the formation of specialized cells (each from one of the daughter cells). As a rule, there are two types of cells in the cambium:

  1. Fusiform initials: These are plate-shaped, elongated cells that taper to a point at the ends, are strongly vacuolated and are called fusiform because of their spindle-shaped appearance. From them emerge the elements of the xylem and phloem, as well as all other cells arranged parallel to the organ axis.

  2. Ray initials: These are almost isodiametric, small cells that are often united in groups. They arise from fusiform initials or their division products. From them, radially arranged medullary rays develop in wood plants (transversal elements).

I. W. BAILEY, who carried out the fundamental work on the structure of the cambium in the twenties, was able to compare a one-year-old and a sixty-year-old strain of Pinus strobus (White Pine) determine a number of relevant data. These values ​​show that both the length of the fusiform initials and their number increase with the age of the trunk. The increase in the number of cells and thus the widening (dilation) of the cambium hollow cylinder is due to the thickening of the centrally located xylem cylinder caused by the cambium itself.

Through tangential (pericline) divisions of fusiform initials, secondary guide elements are emitted in opposite directions, with xylem elements being placed inwards and phloem elements outwards. The derivatives of the initials are arranged in radial rows so that their ancestry can easily be traced back. The regular occurrence of anticlinic divisions of the initials compensates for deficits caused by the increase in the size of the trunk. The activity of the cambium depends on seasonal temperature fluctuations, which leads to the formation of annual rings. Differences in the thickness of the individual annual rings show that the extent of the secondary growth in thickness is influenced by external factors such as temperature, length of the light phase, soil moisture, etc. In addition to the production of xylem and phloem elements as well as parenchymatic cells (in interfascicular sections), the cambium has an important function in wound healing.

This is a secondary lateral meristem that is used to form a secondary closure tissue (bark, phellem or cork). It is issued to the outside world without exception. In many cases, but not always, the phellogen also releases cells into the stem interior, which also form a layer (phelloderm).

Compared to the cambium, the phellogen is quite simply structured. The cells appear rectangular in cross section, flat in the radial direction and plate-shaped in the tangential direction. The plasma is highly vacuolated and it can contain tannins and chloroplasts.

The phellogen arises from cells of the epidermis and / or cells of the parenchymal layers underneath. A distinction is therefore made here between primary and secondary. Phellogenes can therefore be created repeatedly in the trunk. In some species it is single-layered in the first year, later multilayered. It can be active for several years, sometimes even for life, or only for one year. As with the cambium, the activity is controlled by external factors.