UNIT 7:
Secretory Structures

Glandular trichomes exist in various shapes but are often small with a rounded or flattened multicellular apex composed of densely cytoplasmic cells. Secretions (sometimes colored) often fill a space between the glandular cells and a distended cuticle. They secrete a variety of substances from oils and resins to nectars. Some trichomes will be discussed later as nectaries.

1. Menthasp. Make chloral hydrate clearings of leaves for the oil-secreting trichomes. Two types occur: capitate trichomes with one large glandular cell, and peltate 8-10 celled trichomes. Both secrete volatile "essential" oils.

2. Lycopersicum esculentum. Make paradermal sections of leaves and stems and examine for capitate glandular trichomes. Some related Solanaceae produce toxic alkaloids with similar trichomes, some secrete sticky substances that attract insects.

3. Ribes vibanifolium. Examine the large oil-containing trichomes of this species under a dissecting scope. Cavities containing trichomes like these have been proposed as evolutionary precursors to internal secretory cavities.

Latex is an aqueous suspension of substances produced by specialized cells. The suspended substances are often terpenoids (oils and resins) but may include proteins, sugars, fatty acids, alkaloids, starch, and other substances. Large molecular weight isoprenoids from latex are used for making natural rubber. Latex may be produced in idioblasts or in structures called laticifers. There are two main types of laticifers, both are long and narrow and may be branched or unbranched. Non-articulated laticifers initiate as single cells and grow to large size by intrusive growth (pushing their way between other cells). They continue to grow as the plant grows and become giant coenocytic cells. Articulated laticifers begin as a column of cells. Adjoining cell walls sometimes break down during develop-ment. In many species, neighboring columns will anastomose.

1. Euphorbia. The entire family (Euphorbiaceae) contain laticifers.
2. Nerium oleander stems (prepared slide) show good examples of non-articulated laticifers.
3. Lactuca scariola stem contains examples of anastomosing, articulated laticifers. Look at the prepared L.S. view first. Long laticifer tubes occur on the outer border of the outer (external) phloem and possibly within the internal phloem. Find the laticifers in the T.S. based upon their position in the L.S. Stain with Sudan III or IV for lipids and I2KI for starch.

Cavities and ducts are common plant glands that consist of a secretion-filled space lined by an epithelium of glandular cells. The epithelium is often surrounded by an endodermis-like sheath. Epithelium-lined cavities and ducts form by schizogeny-the separation of cells during ontogeny. Lysigenous cavities and ducts are reported to exist where the secretion-filled space forms by lysis of cells. However, there is evidence that many reported cases of lysigneous development are caused by fixation artifacts.

1. . Resin and essential oil cavities and ducts.

   Resins and essential oils are terpene containing secretions produced by plants for de-fense against insects and for allelopathic interactions. Resins and oils are often secreted at the leaf surface by small glandular trichomes-like those of Coleus and Mentha. They are also often produced in spherical cavities or elongated ducts in other species. The smells associated with conifer wood, Eucalyptus leaves, herbs, California Bay laurel, Lemon foliage, and many others, are caused by terpenes produced in these kinds of glands.

   1. Cut T.S. and L.S. through lemon (Citrus limon) peel and draw "lysigenous" oil cavities. Citrus has been classically described as having lysigenous cavities; however, some recent studies indicate that at least some species have schizogenous glands and that problems in preserving oil-containing tissues may have caused a false image of lysigeny. Do you see an epithelium in your freehand sections?

   2. Pinus sp. leaves and stems contain resin ducts. Look at the prepared slides of Pinus from three planes of view. Be able to recognize resin ducts from each view. The epithelial cells surrounding the duct actively secrete resin.

Nectaries are sugar-containing secretions that are common pollinator rewards and attractants for other plant-animal interactions. The three most common sugars in nectars are sucrose, glucose, and fructose; however, other sugars are common, and nectars may contain significant amounts of amino acids, lipids, proteins, and mineral ions. The fragrances produced by many flowers are caused by terpenes produced in oil cells or in specialized oil glands of flowers called osmophores. These fragrances help attract pollinators.

1. Floral nectaries.

   Several common floral nectary types will be provided.

   1. Receptacular disk-type nectaries. This is a common nectary type in dicots. It consists of glandular tissue in a ring on the flower receptacle. It may or may not have a direct phloem supply or stomata for nectar excretion. Examine the disk nectary of the example provided.

2. Septal nectary. Sterlitzia reginal (Bird of Paradise) flower. Make a T.S. of the ovary for the internal palisade epidermis forming a septal nectary.

3. Extrafloral nectaries. Extrafloral nectaries are glands found on leaves and stems that secrete nectar. This is the basis of an important ant-plant mutualism. Ants attracted to the plants remove herbivorous insects from developing foliage, flowers, and fruits. This method of plant defense is especially common in the tropics. Extrafloral nectaries have evolved independently several times from a variety of structures. "Nonstructural nectaries" secrete nectar from stomata with no apparent secretory tissues.

   All other types of extrafloral nectaries lack stomata and usually secrete nectar through an unbroken cuticle (floral nectaries often have stomata). This is possible because cutin is porous when it is not infiltrated with cuticular waxes and oils. Some nectaries consist of aggregates of glandular trichomes-this type is found in all members of the order Malvales. Others are large single trichomes like the scale nectaries of the Bignoniaceae. Others are embedded structures of ground meristem origin. These often have a direct phloem supply, while trichome nectaries often do not.

   1. Examine the micrographs of extrafloral nectaries.
      • Ochroma nectaries consist of aggregates of glandular trichomes. These occur together with pearl glands (not shown) in long grooves on the petiole. Nectar can be seen to drip from these grooves in greenhouse-grown plants. Bombax is a close relative.

      • Observe the photographs in Turner and Lersten (1983). The nectary shown here from the leaf tip of Pomegranate is an embedded nectary, possibly derived from a hydathode. It has a direct phloem supply to the large group of glandular cells.

   2. Chorisia trichome nectaries. This species is in the same family as Ochroma and has similar nectaries in grooves along the abaxial side of the petioles. Make freehand paradermal along the grooves (deep enough to include the bottom of the groove) and observe for nectary structure. How is the nectary constructed?

   3. Ardisia leaves have scale nectaries at the base of the leaflets. Make a clearing or paradermal section that includes the nectaries and observe the nectary structure.

   4. Bull's horn Acacia has an intricate mutualism with several species of ants of the genus Pseudomyrmex. The plant provides food for the ants with the embedded type nectaries along the leaf rachis and the protein and lipid containing modified leaflet tips ("Beltian" bodies). The ants make a nest for their colony by cutting holes in the large bullhorn stipules when the stipules are still young. The hollow stipules then provide chambers for the nest. The ants are aggressive and attack large herbivores as well as insects. The ants also remove seedlings that sprout within the canopy of the tree. The mutualism appears to be obligate. (For more info., see D.H. Janzen, 1967. Coevolution of ants and Acacia's in Central America. Evolution 20(3):249-275).

   5. Passiflora nectaries. All Passiflora species have embedded-type nectaries on the petiole near the lamina base. Nectary morphology varies considerably between species. Some with long stalks are apparently visited by small wasps, others by ants and larger wasps. Compare the nectaries of the species provided for similarities and differences in gross morphology.

Pearl glands are large multicellular lipid storing trichomes that are harvested by ants in a mutualism that is analogous to that described for extrafloral nectaries. Pearl glands are common in the Vitaceae, Piperaceae, Urticaceae, Moraceae, among others.

1. Examine pearl glands on the stems and leaves of Piper nigrum with a dissecting scope. Stain isolated pearl glands or paradermal sections of stem containing pearl glands with Sudan III or IV. The lipids should stain red. Also examine the pearl glands on the petioles of Cercropia sp. in the Urticaceae family.

Hydathodes are sites of water escape when root pressure is high enough and transpiration rates low enough for guttation.

1. Crassula argentea. These secretory structures commonly occur in plant leaves. Take T.S. and paradermal sections of leaves; stain with phloroglucinol. The hydathode consists of a flaired bundle of tracheary elements which connect to small parenchyma cells called "epithem". Find the cluster or continuously open stomata in the epidermis above these hydathodes. Hydathodes in this species are only of the adaxial surface. What is their function?

   B) Mentha piperita. Observe the hydathodes near the tips of leaves in prepared clearings. How do these compare to those of Mentha's relative Coleus

Observe the demonstration of insectivorous plants. These plants use glands to attack, trap, and digest insects. Their specializations seem quite peculiar when they are viewed in isolation. Yet, when we consider the glands of other plants, we find that all of these specialized structures occur elsewhere as well. You saw some of them today. Extrafloral nectaries are used to attract insects. Sticky glandular trichomes catch insects in Drosera (sundew), and they are used for the same thing in some Solanum species, and others. Many nectaries secrete proteins and some glands, like myrosin cells, secrete enzymes. Bromeliads and many aquatic plants obtain their mineral nutrients through absorbtive trichomes. The epicuticular waxes and downward pointing trichomes of pitcher traps are also used by many plants to prevent herbivores and nectar robbers from climbing to flowers and fruits along peduncles of flower clusters. Rapid movements of parts is very rare, but occurs in the stamens of many Opuntia; the leaves of Mimosa; and scattered among other groups.

We will look at the film on insectivorous plants at the beginning of next lab.

PLANT ANATOMY: A STUDY GUIDE AND LABORATORY MANUAL

Thomas L. Rost
Section of Plant Biology
University of California
Davis, California 95616-8537

23rd Edition (1998) Copyright (C); 1975 by Thomas L. Rost
Revised: August, 1998
URL: http://trc.ucdavis.edu/CoursePages/PLB105/HTML/Lab8.html