<p/>Transport, rigidity and leaf shape are aspects belonging to the functions of the venation of a leaf. The function of transport of the free vein endings, last developed within an areole, is suitable for studying the ontogeny of the vein endings, also in case of changes of type of climate. Leaf shape and rigidity which vary little along the <em>Hedera</em> stem, permit such a study.<p/>Examinations of the influence of the climatic conditions on the anatomy of the vascular tissue mostly concern the secondary vascular tissue, especially the wood (Baas, 1973; Carlquist, 1975, 1977). It is important, however, to study the influence of the climate on primary vascular tissue, especially in leaves, because changes of climate likely affect more directly the origin and the development of this primary vascular tissue. For example a change of the rate of transpiration through the stomata does affect the ontogeny of the vascular tissue in the leaf. In submerged plant parts sometimes no xylem occurs in the smaller vascular bundles; if present, the xylem is relatively poor developed (Esau and Kosakai, 1975; Esau, 1975). These observations suggest a reduction of the differentiation of xylem under very wet circumstances.<p/>Since the variability of the structure of vein endings in a leaf is enormous (Cp. Strain, 1933), at first the need of finding some ordering principles in this great variability arised. Pray (1955) remarked 'that some new ontogenetic factor, which previously had not been operative during the differentiation of the earlier developing portion of the venation, has become effective'. These free vein endings originate more independently of, and later than the remaining venation in the leaf development. Vein endings develop progressively from strands delimiting the ultimate areoles (Pray, 1955). An influence of the climatic conditions, notably the relative humidity, will probably be expressed at first in the development of the vein endings. Moreover from the end of the sixties interest arised in the anatomy of the vein endings in connection with the study of the sugar accumulation into the phloem of these veinlets in green leaf tissue (e.g. Geiger and Cataldo, 1969; Geiger, Malone and Cataldo, 1971; Fellows and Geiger, 1974; Turgeon, Webb and Evert, 1975; Turgeon and Webb, 1976; Fisher and Evert, 1982).<p/>When analyzing the structure of the vein endings the basic question arises why during the initiation of the vascular tissue in bundles the differentiation of the xylem turns out to be so strictly connected with the differentiation of the phloem. Would the study of the ontogeny of the vein endings provide more clarity in this matter? Perhaps this provides a contribution to the resolution of the problem: what is a vascular bundle. To take up the problem three questions can be formulated: does the structure of the vein endings in green leaf tissue differ from that in white leaf tissue without chloroplasts, or to what extent a structural response can be perceived to different physiological conditions around the developing vein endings; does the structure of the vein endings, differentiated in leaf tissue in very dry climate differ from the anatomy of those veinlets differentiated in very wet climate, or can the water potential affect the anatomy of the vein endings; and finally the question: can a change in the anatomy of the vein endings in response to different climatic conditions be explained on the basis of the ontogeny, or at which moment in the developmental process the change does appear. This way of approaching to the problem provides insight into the morphogenesis of the vein endings and not only this, but also as to how far this structure is a response to wet and dry conditions.<p/>As experimental plant <em>Hedera canariensis</em> Willd. var. 'Gloire de Marengo' is very suited for trying to answer the questions above-mentioned. This chimeral plant has variegated leaves. The type of variegation is characterized by several shades of green centrally of the leaf and irregular areas of white marginally. Besides totally white shoots did arise of which the white leaves have some small chloroplasts in the epidermis only, especially in the guard cells. This plant is able to grow and develop well in a conditioned growth cabinet in a cooled potometer with an aerated nutrient solution. In the cabinet the long and flexible stems grow unlimited and regularly and can be led easily. These stems form many nearly identical leaves. The leaves have many free vein endings in the mesophyll and these vein endings show a great number of tracheids at their distal ends. The volume of these tracheids does vary experimentally. The leaves of <em>Hedera</em> show a broad structural adaptability under several climatic conditions (Watson, 1942; Wylie, 1943). This change of leaf anatomy under the influence of change of climate can be studied by means of these long stems with many leaves, also during the ontogeny. Finally it turns out to be possible to elaborate the vein endings in the <em>Hedera</em> leaf well microtechnically, and the growth of these vein endings can be expressed well by means of mathematical equations.<p/>The regular occurrence of a distinct maximum of the number of tracheids near the distal extremity of the phloem in the vein endings, that soon became apparent, made one think also of the xylem differentiation being influenced by the differentiation of the phloem, at least under sink conditions. For this reason totally white leaf tissue and later totally white shoots have been used in these examinations.<p/>After gaining an insight into the structure of the vein endings of the <em>Hedera</em> leaf and the specific influence of the relative humidity on this structure, the question of the ontogeny arised, also with a view to a possible more accurately directed, experimental approach to the problem.<p/>Then the question still exists how the venation comes into being and what the cause might be that there is a response to particular conditions. Models are known concerning the venation, and these models can be compared with the results of our examinations into the venation of the <em>Hedera</em> leaf. Physiological stimuli influence the realization of the structure, but these stimuli are tied down to this structure also.<p/>Finally the morphogenesis of the vein endings and the role of possible differentiation stimuli is discussed. In this discussion the different theories of the development of vein endings (as Meinhardt, 1979; Mitchison, 1980, 1981) are involved. In their studies the anatomical structure of the developing tissue is not involved in the theoretical considerations.
|Qualification||Doctor of Philosophy|
|Award date||13 Nov 1985|
|Place of Publication||Wageningen|
|Publication status||Published - 1985|
- vascular system
- hedera helix subsp. canariensis