Para el primer objetivo

Growth and Development in Plants 189

intercalary medstems node

apical meristem

young leaf

internode

node

Figure 10.3:

Intercalary growth at the nodes of stem

It is responsible for the development of the branching system of the stems and roots. The formation of lateral appendages such as roots hairs, leaves and floral parts is linked to intercalary growth. The activities of the intercalary meristem explain why hedge plants and grass shoots regenerate rapidly after being severely damaged in trimming, prunning, lawn mowing and animal grazing. In gymnosperms and most dicotyledons, growth does not only take place by the activities of the apical-root, apical-stem and intercalary meristems but also by the activities of the vascular cambium and cork cambium to bring about increase in the diameter of the roots and stem. This is called

secondary growth

as explained earlier.

Tissues formed as a result of the growth activity of vascular cambium and cork cambium are called

secondary tissues.

Plants which have no vascular cambium and cork cambium (most monocotyledons and a few dicotyledons) achieve increase in diameter of their stems and roots by the activities of intercalary meristems and primary growth, since primary tissues constitute their entire body.

3.3.1 Limited and Unlimited Growths

It is generally stated and accepted that growth is limited in animals while it is unlimited in plants. It has been discovered in human beings that growth in height stops in female at about 17 years and about 20 years in males. This is the instance of a limited growth in animals. Most plants continue to grow year in year out after sexual maturity had been achieved. This is why most plants are seen to achieved. This is why most plants are seen to achieve very great heights and large stem widths. In most plants and animals studied, growth is known to be very fast at the early stages of their life up to sexual maturity after which the rate of starts to diminish. In most plants, growth continues and increase in size and development of new stem and root branches, leaves, flowers and fruits containig seeds are recorded from time to time throughout their life history

3.3.2 Definite and Indefinite Growths

Most animals show definite

growth

while most plants on the other hand show indefinite growth. In segmented animals, the number of body segments is fixed and pre-determined in the embryo. The animal grows that number according to its genetical constitution and never more no matter its longevity or improved living condition. In animals generally body parts are numbered. In tapeworm, however, there is an exc 'on to this rule for the number of proglottides is not fixed or pre-determined in its embryo. It produces proglottides throughout its life cycle as most plants produce leaves. In most plants the number of nodes and intemodes, roots and stem branches, leaves, flowers and fruits containing seeds are not fixed nor pre-determined in the embryo. They are simply being produced from time to time throughout life cycle of the plants. This is an instance of indefinite growth in most plants.

3.4 Growth and Development

So far from the discussion, we can now sum up that growth is a quantitative matter. That means that there is a positive increase in the solid or dry weight and in the amount of protoplasm such that it is measurable with

cell wall cytoplasm nucleus

vacuole

or_ thickening cell wall nucleus

vacuole nucleus

cytoplasm cell wall

root hair

root tip root cap

192 Flowering Plants

Figure 10.4: A longitudinal section through a root tip area of cell division (meristem) cell enlargement and cell maturation

There is no sharp distinction between the regions of elongation and differentiation. Some of the cells could undergo the two processes at the same time. The cells of the region of elongation have stopped dividing but continue to enlarge, and elongate until they are fully matured.

In the region of maturation, the cells grow by strengthening the walls and differentiating into different plant cells. The differentiated cells form the permanent tissues, such as the parenchyma, some of which form the epidermis of the stems and piliferous layer of roots. The majority of other parenchymatous cells form the cortex of the stems and roots in dicotyledonae while in the monocotyledonae they form the ground tissue of the stems and roots since monocots have no cortex. Some other cells become elongated and taper at both ends and they form the collenchyma and sclerenchyma found among parenchymatous cells in the cortex of dicot stems. From the innermost cells arise the central stele (cylinder) of dicot stems and roots made up of endodermis, pericycle, phloem, cambium and xylem. Some parenchymatous cells form the pith at the centre.

Primary growth is restricted to the repetition of the formation of these tissues resulting in the elongation of the roots and stems. This primary growth does not bring about increase in girth or width of roots and stems in dicots because no new tissues are added. The new tissues that are produced, form lateral branches, flowers and inflorescences and are responsible for the elongation of roots and shoots.

Primary growth

is the type of growth in the annuals, biennials, non-woody perennials and monocotyledonae.

Secondary Growth

Secondary growth

does not occur in herbaceous annual or biennial plants. It is growth in thickness due to the division of the lateral meristematic cells which are found between the xylem and phloem. They include the cambium which retains the ability to divide mitotically like the apical cells.

In a young woody plant, the cambium is found between the

_xylem

and

phloem

of the vascular bundle. As

secondary growth occurs, the cambium cells in the vascular bundles will grow radially to link up with the

other cambium cells to form a ring of cambium cells within the stem in between the xylem and phloem of

each bundle. The cells of the cambium ring can divide to produce xylem cells

_inward

and phloem cells

a

ruler or balance. Most multicellular organisms become more and more complex

as

growth proceeds. Thus, there is a marked change in shape, form, degree of differentiation and functions.

The qualitative changes in structure and functions that go on side by side with growth in an organism is called development.

A maize seedling that had grown for two weeks should have got enough fibrous roots to fix it firmly to the soil, enough leaves to carry out photosynthesis and a height of about

twenty

centimetres. The measurable aspects of this seedling include the height and number of leaves and roots referred to as the amount of growth while the qualitative changes in the seedling such as the shape and colour of the leaves constitute development.

Growth

is thus

quantitative,

measurable in the increase in the amount of protoplasm while

development

is

qualitative

observable in the changes in nature of growth and function of the organism. For example, good growth and development of a two week old tadpole larva of toad should have attained a length of about 3cm, respire with internal gills and swim with a well formed tail and tail fin. If it still has external gills with such a large body at that age, it will be said to have grown

without development.

So that

growth

and

development

go on hand in hand. Similarly growth and development cannot be separated, they are two processes that have to go side by side at the same time in living organisms.

Isometric growth (equal growth) and allometric growth (unequal growth)

Growth which is the positive and permanent increase in the bulk or size of the organism strictly follows various patterns in different organisms. A germinating seed or stem cutting first develops long root or roots into the soil or any other suitable substratum before producing the shoot or aerial portion of the plant body.

While in the stem cutting of yam tuber, a long aerial shoot can be developed before any roots are produced.

This is due to the large stored food and mineral salts in the yam tuber. In both plants and animals

the

vegetative and somatic parts are first developed before the reproductive structure. It is clear that

all parts

of the living organisms do not grow at the same time. All the parts of the body that grow side by side at the same time show

isometric

growth. The organs or structures in which rate of growth is different from the rest of the parts of the organism are said to show

allometric growth.

The organs or structures concerned with sexual reproduction are the last to develop. At this time the growth rate of the rest parts of the body has been completed. Thus the main body structures of body plants and animals show

isometric growth

while their sexual reproductive organs shows

allometric growth.

3.4.1 Aspects of Growth Primary Growth

Primary growth starts from the development of

the embryo

into the seedling. This is the growth that forms the main ground tissue of the plant body. It is also the growth that gives rise to the primary root and shoot which continues in the root and shoot apices to develop the main tissues of the plant. It consists of the mitotic cell divisions of the meristematic cells of the root and shoot tips in apical growth and the lateral buds in the intercalary growth.

The growth of plants is restricted to special regions known as

meristems

which may be

apical, lateral

or

intercalary.

The cells at the apex of roots and stems are meristematic cells which are constantly dividing and producing more cells. Behind these cells, are cells which are in a state of

elongation

and maturation and are differentiating into vascular tissues, epidermis, parenchyma and sclerenchyma cells.

Thus, the tip or a growing point of a stem or root can be divided into three regions; a region of mitotic cell division followed by a

region of cell

elongation and then a region of cell maturation and differentiation.

This is the same for roots and stems. see fig. 10.4

Growth and Development in Plants 193

outside. During secondary growth, the cambium grows to, form a ring, connecting all the vascular bundles together. The cambium ring then begins to produce both secondary xylem and secondary phloem tissues.

Outcome of Secondary Growth

Secondary growth gives rise to new xylem tissues on the inner side of the cambium and new phloem tissues are added to the tissues formed during primary growth in the first year. The new xylem is pressed against the older xylem towards the pith. Thus the cells of the older xylem (protoxylem)look crushed. Their lumens are greatly reduced as a result. The new phloem tissue formed is pushed outwards exerting pressure on the older phloem towards the periphery of the stem. At this time a cork cambium is formed just below the epidermis from some cells of the cortex in stems of dicot. The cork cambium becomes meristematic, dividing to form secondary cortex to the inside and cork cells to the outside. The cork cells become dead and impermeable to water to form the bark of the stem. In some areas of the bark, openings called lenticels are formed, through which respiratory gases are exchanged.

Primary growth is the main growth in the first year in the tropics and first growing season in the temperate regions of the world. Secondary growth starts in the second year. In each growing season cambium produces a ring of xylem towards the inside of the stem and a ring of phloem towards the outside of the stem. This is referred to as an annual ring to growth ring. Annual rings or growth rings can be counted to determine the age of the plant since a ring of growth is formed every year. Annual rings are very conspicuous in temperate trees because of the distinct summer and winter.

In your own words, briefly explain the three major basis of growth 2. Write short notes on the following types of growth

i. Intercalary growth

11. Limited and unlimited growth iii. Definite and indefinite growth

3.5 Measuring Growth

Growth is quantitative and measurable so that the amount of growth achieved by plant can be expressed.

The known and widely used measures or indices of growth include;

1. Height or length increase of the stem, root and any other organs of the plant.

2. Increase in the girth or circumstances of the stem 3. Increase in the area of leaves

4. Increase in weight and

5. Increase in number of stem branches.

The foresters and timber collectors are interested in the increase in height and circumference of the stems of forest trees. The practical farmers are interested in the increase in the volume of fruits and increase in the fresh weight of vegetable leaves, stem and root tubers. The practical teacher and research students are interested in the increase in length of root and shoot of young seedlings and increase in number of individuals in a population. The amount of growth achieved by a seedling at 24 hours interval over a known period of time represent the rate of growth. The increase in height (mm) can be plotted on a graph over time intervals (days). The rate of elongation or height growth is seen to vary from individual to individual and from species to species. The graphs are the same. In all cases, the rate of growth is at first slow, then rises up to a point and continues at this rapid rate until maturity, when it declines. The graph shows a typical S-shaped curve called sigmoid growth curve. The sigmoid growth curve is characteristic of all living organisms.

In document “Estudio de las condiciones ambientales y sistemas de manejo de cérvidos (Cervidae) en cautiverio en Ecuador” (página 56-61)