SECRETORY TISSUES

SECRETORY TISSUES
The group of cells usually of different origin concerned in secretion of
oils, nectar, resins, gums, mucilage, latex etc is generally termed as
Secretory tissues.
Secretion means the process of release of by-products of
metabolism from a cell. The secretory substances may also be
stored in insoluble forms within the cell. These substances have
either a special physiological function or may be secreted or
stored as wastes.
The removal of waste products of metabolism is defined as
excretion.
Secretion is generally used to describe the various processes by which
individual cells or multicellular structures perform any of the functions
mentioned below:
 Remain alive and discharge their products outside the plant body or
into special nonliving ducts or cavities within the plant body.
 Undergo lysis and rupture, releasing their contents, often into a duct.
 Store the product within the living cell or between protoplast and wall
of the cell.
The secretory bodies vary greatly in structure and position. They are
mainly of two types:
1. The Secretory Cells in which the secretory substances are formed
and exuded outside the cell e.g., simple glandular trichomes,
nectarines, etc., and
2. The structures in which the secretory substances are stored and
released after the breakdown of the cells. The secretory structures
may be external and internal.
Figure: Electron micrograph of a maize root cap slime-secreting cell showing
two Golgi stacks in the centre. These are surrounded by large white sacs filled
with mucilage produced by the Golgi apparatus.
Mechanisms of Secretion
There are several methods by which the elimination of secretions from
the protoplasts takes place. Most prevalent of these are:

Holocrine Secretion
In holocrine secretion substances are
released as a consequence of cell
disintegration. The entire cell contents
form part of the secretion and the
product is liberated by complete
breakdown and disintegration of cell.
Cell lysis may be accompanied by
elaborate alterations in the cell
organelles.
Merocrine Secretion
In merocrine secretion, protoplast releases substances through its
intact plasmalemma or tonoplast.
Classification of secretory structures
The classification is based on the type of secretory structure involved in
exosecretion (substances discharged onto the external plant surface) and
endosecretion (substances discharged at an internal site or stored in the
protoplasts) and also the nature of the secretory products.

External Secretory Structures

Glandular trichomes
 Trichomes are unicellular or multicellular appendages on epidermis.
 Trichomes include hairs, scales and other structures and may be
glandular or stinging types.
 Glandular trichomes are involved in secreting various substances such
as water, salts, nectar, mucilage, terpenes, adhesives, digestive enzymes
and irritants that sting.
 Almost all glandular trichomes possess a unicellular or multicellular
head composed of the secretory cells usually borne on a stalk of
nonglandular cells called endodermal cells.
 The endodermal cells prevent the secreted solutions from coming
back into the plants through the apoplast.
 The secretory cell usually contains large number of mitochondria
and other organelles.
Schematic drawings of different
types of glandular trichomes of
Cordia verbanacea leaves. A-D.
Reniform trichomes. E-F.
Globular trichomes. The
number of stalk cells varies in
both globular and reniform
trichomes, but the upper stalk
cell is always distinct, like a neck-
cell.
Transverse section of stem of foxglove (Digitalis pupurea) showing
glandular trichomes.
Scanning electron micrographs of
A. Glandular and non-glandular trichomes of tomato (Solanum lycopersicum).
B. Two types of glandular trichomes on Cannabis leaf surface: capitates (having stalks)
and peltate (rounded and closer to the leaf surface).
C. Clary sage (Salvia sclarea) showing stalked and sessile secretory glands on the calyx
trichomes.
D. Glandular trichome of Artemisia annua.
Hydathodes: Trichomes secreting an aqueous solution
Hydathodes are special pores present on the leaf epidermis that release
liquid water having dissolved organic and inorganic substances, by a
process called guttation. When transpiration is minimal, and the root
pressure causes xylem sap pressure to be high, the water exudes through
these pores.
Cells of hydathodes are small, thin walled with dense cytoplasm and
are devoid of chloroplasts.
Hydathodes are of two types:
(i) Epidermal hydathode: the epidermal hydathode secrete ions and
minerals along with water. They are also referred to as Salt glands.
(ii) Epithem hydathode: are referred to as hydathodes or water stomata.

A typical hydathode consists of

(a) water pore,
(b) epithem and
(c) tracheids

Hydathodes have been reported in the leaves of over 30 genera and 115
families especially grasses. Some tropical plants like Colocasia can guttate
upto 100 milliliters of water in one night.
The hydathodes of the leaves of Populus species are considered
unspecialized since they guttate water containing various concentrations
of sugars known as nectar.
The trichome hydathodes are active hydathodes that secrete salts and
other solutions that may cause injury to plants through accumulation and
interaction with pesticides.
Salt Glands and Chalk Glands: The Salt secreting trichomes
Salt glands generally occur in plants that grow on saline soils (halophytes),
and secrete inorganic salts.
 The secreted solutions of salt glands mainly contain the Na+ and Cl– ions.
Other ions secreted by salt glands are K+, Mg2+, NO3-, PO43-, Br– and HCO3-.
 Many salt glands are similar to hydathodes in the sense that the main
source of secretions is the transpiration stream.
 However, many of them differ from the hydathodes with respect to their
origin.
 Multicellular trichomes adapt to perform glandular functions, whereas
hydathodes have stomatal origin.
Many of the salt glands show characteristics that can be associated with a
sophisticated secretory system such as
 The means of absorbing the material to be processed over a large
surface area.
 The means of transporting it within the glands (transfer wall or fields
of plasmodesmata).
 A discrete group of cells specialized for the actual secretion.
 The means of isolating the secretion and preventing it from leaking
back into the symplasm.

Salt glands occur in at least 11 families of dicotyledons and in one family

of monocotyledons, the Poaceae.
Salt secreting trichomes are mainly of two types:
Bladder-like hairs called as Salt Bladders:
 Salt glands in the form of salt bladders are the characteristic feature of
family Chenopodiaceae that includes the Atriplex (salt bush) species.
 The salt glands of Atriplex consist of a narrow stalk cell on the top of
which is present a large secretory bladder cell.
 The bladder cell consists of a large central vacuole at maturity. A cuticle
layer covers both the bladder and stalk cells.
 The bladder cell and the mesophyll cells of leaves remain connected
through symplasm.
 The ions of transpiration stream are delivered to the bladder cells with
the help of protoplast and plasmodesmata.
 The bladder cell secretes these ions to the central vacuole and
eventually lyse, and salt gets deposited as a white powdery layer on the
surface of the leaf. Therefore, the mode of salt secretion is holocrine
secretion.
Glands that secrete salts directly outside
 The two–celled glands of Cynodon, Spartina, Sporobolus and Distichlis
are considered to be the simplest glands that secrete salts onto the
surface of the secretory cells.
 These two celled glands consist of a basal cell and a cap cell.
 The cap cell and adjoining epidermal cells contain a continuous cuticle
cover. Both the cap cell and basal cell lack a large vacuole.
 The cap cell contains a large nucleus.
 Between the outer secreting and inner non-secreting cuticle there is a
collecting chamber.
 The basal cell is characterized by the presence of partitioning
membranes (extensive invagination of plasma membrane) that play a
vital role in overall secretion.
 Therefore, the type of secretion in the two celled glands of Poaceae is
eccrine type.
 The uptake of ions from the basal cell and its secretion from the cap cell
in the salt glands of Sporobolous, involves ATPase activity.
 Both these processes (uptake and secretion) are considered to be active
processes.
 The symplastic continuity between basal cell and cap cell as well as
between basal cell and neighboring mesophyll cells is maintained by
plasmodesmata.
Osmophores: The fragrance producing glands (Gk. osmo = odor, and
pherein = to bear)
 The special glands that are involved in producing fragrance in some
plants are called as osmophores.
 The volatile substances such as terpenoids and aromatic compounds
are responsible for production of fragrance in flowers.
 Osmophores are found in a large number of families including Araceae,
Asclepiadaceae, Orchidaceae, Solanaceae, Aristolochiaceae, Saxifragaceae
and Burmanniaceae.
 In some sps of Araceae, the fragrant substance may contain amines and
ammonia in addition to terpenes.
 The osmophores appear as flaps, cilia or brushes and can be stained
with neutral red.
Crystals and silica secreting cells
 Many plants deposit inorganic materials mostly calcium salts and
silicon oxides in their cells.
 The calcium salts form crystals and silicon oxide forms silica bodies.
Crystal-containing cells are often treated as secretory idioblasts.
 Different types of crystals are:
1. Idioblasts: They are stellate calcium oxalate crystals usually found in
the aerenchyma of aquatic plants to give support to the tissue. They
are quite common in Limnanthemum, Nymphaea, Trapa, etc.
2. Prismatic crystals: They are single calcium oxalate crystals found in
various plants. They may be square, prismatic, rhomboidal or
pyramid-like in shape.
3. Raphides: They are needle-like, long slender crystals usually lying
parallel to each other in a bundle, which is sometimes found in a special
sac like cell. When the sacs are injured mechanically, the raphides are
released out with a jerk through a small hole. They are commonly found in
Alocasia, Colocasia, Pistia etc.
4. Druses, rosette, crystals or cluster crystals: This is one of the
commonest compound crystal having the appearance of a rosette and
known as rosette crystal. They are common in Eucalyptus, Nerium, Ixora
etc.
5. Sand crystals: They are commonly found in masses of micro sphenoidal
crystals packed into a cell. Such crystals are generally found in the leaves
and roots of Atropa belladona of Solanaceae.
Silica is deposited mostly in cell walls, but sometimes it forms bodies
in the lumen of the cell.
Cystoliths:
 The crystals which are composed of calcium carbonate are commonly
known as Cystoliths.
 The main body of cystolith is a cellulosic extension of the cell wall in
which the calcium carbonate is deposited in the form of fine granules.
 They project into the protoplasts in large specialized cells.
 These str are lime-encrusted or lime-infiltrated, stalked projections of
the wall.
 The foundation of the cystolith is a stalked stratified, cellulosic body
which arises early in cell dev as a local wall thickening.
 With the addition of large amounts of calcium carbonate this becomes
an irregular body which may nearly fill the cell.
 They are generally found in Moraceae, Urticaceae, Acanthaceae,
Cucurbitaceae, etc.
Laticifers:
Laticifers are the specialised cell or row of cells that secrete the milky or
watery fluid termed latex.
It forms various str like latex cell, latex vessel, latex duct, latex tube and
laticiferous duct.
The laticiferous duct is a tubular cavity into which latex is secreted and
remains stored.
A single vegetative cell may be converted into a simple or branched latex
cell.
The latex ducts are also modifications of the vegetative cells into
aseptated, elongated and branched str.
Based on origin the laticifers may be
1. Simple: they are derived from a single cell
2. Compound: these are originated from a longitudinal pile of cells

The cell wall of the laticifer is non-lignified but thicker than the adjacent
cells and they are composed of cellulose, hemicellulose and pectin.

Chemical composition: It contains sugars, proteins, alkaloids, oils,

mineral salts, organic acids, terpenes, resins, rubber etc.
Articulated and non-articulated laticifers:
 The non-articulated laticifers are derived from the enlargement of a
single cell, has the potentiality of unlimited and rapid growth, and
elongates to form a long unbranched latex tube.
 In some plants the non-articulated latex tubes may be branched.
 They are coenocytic and multinucleate, and also termed as laticiferous
cell.
 The laticifers grow through the intercellular spaces with the help of the
enzyme pectinase secreted by the growing tips of the laticifers.
Articulate laticifers or laticiferous vessels:
 They consist of longitudinal pile of cells.
 The transverse end walls of the individual cells may remain intact or
partly or totally obliterated to form continuous tube called the latex vessel.
Therefore, they are of compound origin.
 They occur in primary or secondary phloem and in cortex.
 The articulated laticifers may remain as a single chain of cells without
anastomosis (articulated non-anastomosing laticifer, eg Allium, Musa
etc.).
 They may also form a complex anastomosing system called Articulated
anastomosing laticifers (eg. Carica papaya Papaver etc.).
 The enzyme cellulase is found in the latex of articulated laticifers
suggesting that it may be involved in the lysis of common transverse walls
during development.
Nectaries: Nectar - secreting trichomes
Nectar (an aqueous fluid with a relatively high concentration of sugars) is
secreted by specialized secretory structures called nectaries. Nectaries are of
two types:
Floral nectaries: occur within flowers and are directly associated with the
pollination process.
 Pollinators like insects and other animals are rewarded through the
secretion of nectar by the floral nectaries.
 Different organs of a flower like sepals, petals, stamens, ovaries or the
receptacle may bear floral nectaries.
Extrafloral nectaries: occur on vegetative structures or on peripheral
floral parts and are usually not associated with pollination or
reproduction.
 They function in attracting insects, primarily ants, which by their
presence and actions protect the host plants against grazing mammals
and foliage eating insects.
 Some extrafloral nectaries serve as safety valves for releasing excess
sugars when photosynthetic activity is especially rapid.
Figure: The nectary trichomes in Abutilon megapotamicium showing (A) flower,
(B) nectar hairs with globular heads just before secretion, and (C) hairs with
drops of nectar on the head cells.
Colleters: Multicellular appendages that produce sticky secretions
 Colleters are multicellular appendages that appear mostly on young
leaves and buds and secrete glue like sticky secretions.
 The nature of this sticky secretion is mucilaginous or resinous and is
insoluble in water.
 The colleters probably protect the dormant buds, developing
meristems, differentiating leaves by giving them protective coating.
 The standard type of colleters found in the family Rubiaceae is the
most common type.
Figure: Sundew (Drosera sp.) showing multicellular trichomes known
as colleters which trap insects.