Developmental plant anatomy must include studies that focus on key aspects of plant structure-function relationships. The material that is presented in the 'Factfiles' supplements that which can be found in Applied Plant Anatomy, and students are encouraged to cross-refer and to make use of The Anatomy of Seed Plants and Plant Anatomy by Esau, as well as any other plant anatomy reference texts that are available. The material covered in the Factfiles includes leaf development, the development of the vascular system, intercellular transport, phloem loading and unloading. Interest will be focused on the leaf and specifically, development of the xylem and phloem.
Early life forms were single-celled. Life may have been in simplistic terms comparatively 'easy' as each individual controlled its own destiny -- manufacturing energy sources, developing efficient respiration and light-harvesting complexes, which made the system more efficient in terms of photosynthesis and growth capacity. At about this time, the need to develop storage capacity became a priority, as single-celled systems have severe limitations on their reserve storage capacity. The next logical step was the evolution of multicelled systems. Specializations and evolutionary advances could manifest themselves. Cellular specializations - cells 'designed', if you like, and purpose-built for specific tasks within the living system. Increasing size and complexity of form resulted in the development of more complex, less and less motile and partly water-independent organisms. Today, we understand multicellularity to involve many things, including processes whereby aggregated cells required and evolved cell to cell transport pathways. Intercellular communication occurs via finite control pathways, through regulated (gatable) plasmodesmata.
The development of larger multicelled and then supracellular organisms allowed the development of compartmented structures, in which regions of the organism evolved specific roles and functions. Some of these would have been specialized for reproductive purposes, but, for the most part, plants remain predominantly vegetative. Biochemically, all plants function as a series of sources and sinks - with sources being the sites at which essential carbon skeletons are synthesized and sinks being those regions where these carbon skeletons are either accumulated (stored) or metabolized and converted into other more useful secondary plant metabolites associated either with growth or further development of the plant. Take for the potato tuber as an example.
A good example of multiprogrammed source sink Interconversion can be seen in the potato tuber. The tuber acts as a strong carbohydrate sink in the summer season, attracting excess carbohydrates which are converted into starch. Once the potato plant has senesced and died, the tuber will undergo a series of complex changes, related to the onset and formation of "eyes" (root primordia). This occurs particularly in relation to its transport physiology, with the result that the tuber becomes a carbohydrate source to the young developing plant. According to Oparka et al., (1992) unloading from the source occurs via plasmodesma and the sucrose is stored in the vacuole. Starch synthesis in amyloplasts takes place, lowering the osmotic potential, thereby maintaining source to sink flow. Loading from the source must reflect the reverse process as the 'eyes' sprout, the tuber's becomes the source of the initial carbohydrate supply needed at the onset of growth.
Sucrose transport is an active, turgor-sensitive mechanism in the potato tuber, which will result in the retrieval of sucrose escaping to the apoplast. Oparka et al., (1992) postulated that the plasmodesmata are closed during sieve element loading, in other words, phloem loading follows an apoplasmic pathway. The potato tuber clearly is not a simple system - but one which changes from being an active sink to an active source of carbohydrate (according to the season) and one in which the plasmodesma appear to be able to undergo selective opening and closure (referred to as gating in the literature).
Evolution into multi- and supracellular organisms has meant increased complexity of form. The study of complexity of form can involve many sub-disciplines of Botany, of which plant anatomy is core to understanding the processes that are going on all the time. Structurally, we could focus attention on root, stem or leaf, and get a good idea of plant structure function relationships.
Why study the leaf?
Leaves are the primary site of carbon assimilation via photosynthesis. They are important therefore, in terms of the quantity and quality of assimilate produced. Photosynthesis in leaves is formed in source leaves where the assimilate is uploaded into the phloem and is transported to regions in the plant where it is either stored or used. Storage or re-use occurs in structures that are termed sinks, where assimilates are unloaded from the phloem. Transport is rather complex between source and sink. It is fitting therefore, that we explore the leaf in terms of its ontogeny, its structure and modifications as well as paying attention to biochemical structure-function relationships. We will examine the pathway(s) followed by assimilates from the primary photosynthetic sources (palisade and spongy mesophyll) to the functional accumulating and long-distance transport, and unloading phloem in higher plants.
One of the major problems in teaching an advanced course, is that there is no single text book that covers all the material that needs to be read or referenced. Esau's Plant Anatomy and The Anatomy of Seed Plants form the core of this course. Just as important, are Fahn's Plant Anatomy and the revised version of Applied Plant Anatomy (Cutler and Botha). Each of these texts has their own specific approach - Fahn is particularly useful for example, if your interests are centered round dry to desert conditions, whilst Esau tends to look at many examples, mesophytes. No single book is sufficient to cover all aspects of plant anatomy, but many aspects and focus areas are missing from general texts, and one has to focus attention on specialist texts and peer-reviewed research articles.
Because of the difficulties and limitations mentioned above, we have prepared a series of short essays which, hopefully, will provide additional useful core material. These are, for want of a better term, called "Factfiles" , Many topics of importance to your understanding of this course are covered in a series of factfiles. These have been prepared as hypertext documents that you may access directly. We will explore plant growth from the apex (#1 Vegetative apical growth) where we will explore the control mechanisms that are involved in apical differentiation; we will look at aspects of the functional relationships in leaves (#2); leaf anatomy (#3); morphology and tissue systems (#4); the source-sink connection (#5); plasmodesmal structure, function and role (#6); cell-cell communication in plants (#7); phloem loading ecophysiology (#8); plasmodesmal modifications (#9); the evolution of the supracellular organism (#10); phloem transport mechanisms (#11). Some basic information concerning fixation and embedding techniques have been included, principally for the preparation of material for light microscopy (#12) has been included, as have some common histochemical techniques (#13).
Whilst not definitive in all respects, we believe that these Factfiles serve a purpose - they provide easy to access information, which, we hope, will stimulate greater interest and awareness in plant anatomy. Our core objective is simply to allow easier integration and absorption of (useful) of facts. We hope you find these Factfiles useful and that they help you integrate structure and function more clearly. There is a great deal to learn about the interrelated form and function in plants
Oparka, KJ, Viola, R, Wright, KM, and DAM Prior. 1992. Sugar transport in the potato tuber. In: C. Pollock, J Farrar and T. Gordon eds. Carbon partitioning within and between organisms. Bios Scientific Publishers, Oxford.