An introduction to functional hydrophyte and mesophyte structure.

 

Note: The information presented in this web page is supplementary to any of the good text books available to you in the Library or to your currently prescribed Botany text book. The information is intended primarily to be a study aid, NOT to enable you to do away with reading text books.

Life in water requires that the plant is either submerged, and free-floating, anchored to the substrate, anchored to the substrate, or with the upper leaf surfaces exposed to the air. Hydrophytes range in size from the very small Wolffia which is the smallest flowering plant, with leaves only 1mm wide, to the Royal Water Lilly Victoria regina which has leaves 1 to 2 meters in diameter!

In each case, modifications are required to enable the day to day life of the plant (and particularly, its physiological processes)  to occur at somewhere near optimum for that species. For example, nutrient supply for floating hydrophytes, such as Eichhornia present few problems, as the plants, being natural drifters, will find nutrients wherever they float to. Free floaters have masses of fine-netted roots which help accumulate the nutrients needed for growth. They have very high transpiration rates.

 

Victoria regina the Royal water lilly - note the massive globular protruding rib-like structures (the major veins of the leaf)  which support the otherwise huge, thin leaf blade.

Hydrophytes such as Eichhornia crassipes  (above) float, due to the presence of bladder-like structures -- they are a major pest in many sub-tropical and tropical waterways. They were introduced into the United States of America at an horticultural exhibit in Louisiana - clogging waterways here and in other areas such as Africa and India present major problems, as they transpire at a very rapid rate -- Water hyacinth-covered ponds loose water up to eight times faster than uncovered ponds. In its natural environment -- the Amazon, it does not present a problem as there are enough herbivores to eat it.

Remaining afloat is one of the major problems in an hydrophytes life - gaseous exchange too, is important, In order to facilitate these functions, hydrophytes tend to develop large intercellular spaces, which may be subdivided to prevent free movement of large air bubbles, by forming complexes of cells, which become effective  bubble barriers.

Elodea has a large proportion of the stem occupied  by structured airspaces, which are separated from one another by nodal plates.
The illustrations on the right, show some common forms of embellishment, to enhance bubble trapping.

Stems such as Potamogeton show remarkable formations of aerenchyma cells form dissecting the stem up into 5-6 sided zones -- these will effectively trap large air bubbles and will assist in floatation and gas exchange.

 

Hydrophytes maintain buoyancy by developing intercellular spaces that can trap gas bubbles.

Whilst obtaining enough water is not a problem, gas exchange and light intensity do present problems under some conditions.  Submerged leaves have reduced gas exchange capacity (diffusion through water is slower than through air) and the quality and quantity of light reaching submerged leaves falls off dramatically as one descends beneath the water surface.  Most of the leaf modifications focus on enhancing light absorption and gas exchange.

 

Submerged aquatics have a variety of leaf forms -- study the diagrams below and this becomes clear.

What kind of advantages do these forms of leaf present?

Anchored Hydrophytes

Anchored hydrophytes  have rooting systems embedded in the substrate and usually have floating leaves.

Here we see the central vascular bundles within the petiole of Nymphaea.  Notice that the protoxylem poles face each other - the two bundles are recurved toward each other). Note also the large intercellular spaces.

Supporting tissues are limited, as there is very little need for this 'expensive' tissue. Note the dark red star-shaped structures in the photomicrograph above - there are sclerieds which serve very little purpose except in the lamina or leaf blade.

Leaf shape varied in the same plant, between submerged and aerial leaves.

HETEROPHYLLY

Condition where the same organ has a change in form. The submerged aquatic leaf is simple, (upper diagram) and only three cells thick, whilst the floating leaf (lower diagram) contains numerous intercellular airspaces and has a columnar mesophyll arrangement. 

Vascular supply is provided by veins, containing xylem (upper side) and phloem tissue (lower side).

 

Mesophytic Plants

Mesophytes have to contend with a number of issues which will directly and indirectly exert effects upon their physiology. For example, high light intensities, diurnal temperature ranges, water stress and nutrient status are perhaps some of the most important. Light intensity, temperature and water availability will govern the day to day life of the plant and will affect the structure (morphology and anatomy) of the plant itself.

Of interest is that fact that the vascular system has to become well developed in order to ensure survival. ( see The making of plant vascular systems -- from single cell to supracell) web page for more information on vascular development).

 

 

The central part of the stele in Selaginella (above) contains a central core of xylem, (green in this illustration) surrounded by phloem tissue. Cycads have relatively well-developed vascular tissue. Being Gymnosperms, they have tracheids only as the water conducting cells, and sieve cells serve for long-distance transport of carbohydrate.
Vessels, tracheids, sieve tube members, companion cells, sieve cells and albuminous cells -- these constitute the transport pathway within vascular plants.

Vascular development is various and in some instances, very complex. Wood formation for example, is governed by the climate prevailing where the specimen grows -- the wood 

WOOD ANATOMY

 

The 'key' to functional state and indicator of water availability

 

TROPICAL  WOOD    has large-diameter vessels, whilst TEMPERATE WOOD tends more towards smaller diameter conducting elements, and some may show seasonal size change (large in spring, smaller in autumn).

Phloem becomes a highly specialized conduit for the transport of dissolved sugars, that are carried in an aqueous phase. In Cucurbits (pumpkins) one finds beautiful sieve plates which are visible even with relatively poor microscopes. Look at the structure on the right it looks just like a sieve right? Its role is to retain the functional content (mitochondria and associate endoplasmic reticulum) in place. If the cell is damaged, then callose is deposited quite rapidly over the sieve plate pores.

 

 

Concluding remarks.

The information which I have provided you with will hopefully, awaken an interest in the wonderful and exciting structures we can see with the microscope, which reveals a myriad of sometimes exquisite cellular structures and arrangement of cells in plants. I hope you enjoy looking at the examples I have given here, and at those in the text books, and that you never cease to wonder at the microscopic marvels hidden within plants. 

Please make sure to read the Text Booky Moore Clark and  Vodopitch (The prescribed text for 1999)  Especially important are:

CHAPTER 28 Seedless Vascular Plants p 678; CHAPTER 29 Gymnosperms p 706; CHAPTER 30 Angiosperms p 732;and CHAPTERS 13-15, where you need to concern yourselves with the basics of STEM and ROOT  structure only.

You are well-advised to study other texts, including the latest edition of Raven Evert and Eichhorn 'Biology of Plants'  (6th ed.)   and in particular, to look closely at Chapter 1 Botany: An Introduction and Chapters  17-22, which deal with lower order land plants and the evolution of Angiosperms.

Enjoy!

 

If you have any useful comments, then please contact t.botha@ru.ac.za I'd like to hear from you!