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Certainly, no single hormone-based model can be presented at present to account for sexual different- iation. Studies of protein patterns and other gene- product analyses are rare and at present inconcl usive. HALEVY, ABRAHAM H. Department of Environmental Horticulture, The University of California, Davis. (on leave from The Hebrew University, Rehovot, Israel). - Regulation of petal senescence. The main growth regulator controlling petal senescence of some flowers (e.g., carnations) is ethylene. Other flowers (e.g., roses) are much less sensitive to ethylene. The most effective inhibitor of senescence and abscission in ethylene-sensitive flowers is Ag+, applied as Ag-thiosulfate. Cytokinins delay petal senescence by inhibiting ethylene biosynthesis. Pollination promotes petal senescence. There seems to be a multi-stage pollination-induced senescence signal. The first one being I-aminocyclopropane-lcarboxylic acid (ACC), diffused from the pollen. Wound-ethylene and auxin may participate in later stages. In both ethylene sensitive and ethylene nonsensitive flowers, a decrease in membrane fluidity was observed during aging. Environmental or chemical agents modifying the rate of senescence, correspondingly also altered the rate of change in petal membranes fluidity. The decrease in fluidity corresponding to an increase in the ratio of free sterols to phospholipids, due to a decrease in the content of membrane phospholipids. The activity of petal ATPase and of sucrose uptake by the petals is correlated with membrane fluidity. These processes may regulate the reduction in water and dry weight content of fading petals. KEVAN, PETER G. Department of Environmental Biology, University of Guelph, Guelph, Ontario NlG 2W1, Canada. - Floral characteristics for pollinators. Floral attractants appeal to the senses of pollinators and should be investigated with those in mind. Attractants may be visual, as colors, color patterns, sizes, outline shapes, and arrangement of floral parts. Depth effects, as in tube- and funnel-shaped flowers are also involved. Olfactory cues function in long- and short-distance attraction of, and in flower discrimination by pollinators. Guide patterns on flowers may be visual, chemical, and structural and aid pollinators in foraging and pollinating. Mi- crosculptural features on flowers are distinguished by insects which may use them as guides and in plant species recognition. Some floral structures benefit pollinators by providing shelter and/or warmth. Color, form, presentation, and sporophyll placement indicate the various pollination syndromes. Correla- ted with those are features of pollen and the chemical nature and amounts of floral rewards for pollinators. KONING, ROSS E. Department of Biological Sciences- Botany, Rutgers University, Piscataway, NJ 08854. - Plant hormones and growth of flower parts during flFoweropening. There is no simple understanding of the roles of plant hormones in control of flower part growth since, until recently, no species had been examined for the Developmental and Structural Section 11 roles of all of the plant hormones, both applied and endogenous, in growth of all of the flower parts. Such an analysis was recently completed for Gaillardia gra randiflora. Three types of measurements weiMade: 1) growth of the flower parts under natural conditions, 2) growth of isolated flower parts in response to hormones applied in vitro, and 3) the endogenous hormone levels. These measurements were correlated with each other to determine the roles of the various plant growth substances. In Gaillardia, corolla elongation occurs by gibberelliT ofrel, ffilament and style elongation occur by auxin control, and stigma unfolding occurs by ethylene control. The ray flower corolla must be fully expanded to attract pollinators before the fertile disc flowers open, thus explaining need for separate controls. The growth of the filaments and the style can be controlled by a single hormone; the sequential events are timed by the position of the two organs with respect to the auxin source: the pollen. As pollen develops, auxin is first transported to the filament; only after the pollen is shed directly onto the stigma, is the auxin transported to the style. The level of auxin in the stigma is then high enough to promote the production of ethylene and cause stigma unfolding. The tidy control mechanism elucidated in Gaillardia probably applies only to members of a sma of Asteraceae species, since in the literature, unrelated plants have different methods of developmental control. Future complete studies will provide a basis for sorting the seemingly contradictory findings into a more systematic understanding of flower part growth. LANG, ANTON. MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824 - Flower induction: endogenous hormone-like factors . Evidence for a hormone-like inducer of flower forma- tion ("florigen"), which is formed in the leaves while acting in the apical shoot meristems, exists for about 50 years. Most of it is based on grafting experiments between photoperiodic plants maintained in the noninductive daylength ("receptors") on one hand and similar plants exposed to the inductive daylength or dayneutral (DN) plants ("donors") on the other. Experiments of this kind have been done in eight plant families. It has al so been shown that long-day (LD) receptors can be induced to form flowers by LD, short-day (SD) and DN donors; SD receptors can be induced by SD, LD and DN donors; and flower in DN receptors can be hastened by LD and SD donors. Donors and receptors can belong to the same species, to different species of the same genus, and to different genera. Thus, florigen appears to be ubiquitous and to be nonspecific in both the taxonomic sense and with respect to the physiological response type. For about 5 years we also have evidence for a graft-transmissible inhibitor of flower forma- tion ("antiflorigen"), so far in two LD plants. Antiflorigen inhibits flower induction in DN and SD plants of the same or a different genus; like florigen it is evidently nonspecific both taxonomically and physiologically. Flower induction thus appears to involve both promotive and inhibitory, endogenous hormone-like factors: florigen which in LD and SD is formed only in LD and SD respectively, while in DN plants it is formed independent of daylength; and antiflorigen which is formed at least in some LD plants under SD conditions. This situation will be briefly discussed in relation to other aspects of f lower i nd ucti on .
12 Developmental and Structural Section Symposium: Homology in Modular Organisms-Concepts and Consequences INTRODUCTION It is proposed that the concept of homology as it applies to modular organisms (specifically higher plants) be the subject of this symposium. The dis- cussion will be timely as the recognition of homologous structures is foundational to the application of cladistic methods. If the basic assumptions of cladistic analysis can be shown to be well-founded, its methodology is strengthened; if it is not, then the methodology is likely to lead to erroneous conclusions. Clearly also the basic concepts of evolutionary morphology of higher plants deserves a careful scrutiny independent of any influence derived from the study of organisms with unitary construction. Organized by P.B. Tomlinson, Harvard University, Petersham, MA. KAPLAN, DONALD R. Department of Botany, University of California, - The problem of serial a case study. Berkeley, CA 94720 homology in higher plants: When two different organ forms are produced along the higher plant shoot, intermediates between the two types typically are formed at the intervening nodes. Such transitional forms have been a principal source of evidence in determining the structural correspondences (serial homology) between the two leaL types. Typically most morphologists in the past were content to determine such structural relationships from the comparison of fully-developed organs rather than study their development. In a recent study of the developmental basis for the classic case of heterophylly in Acacia it was shown that such deductions of serial homology based on mature leaf forms led to erroneous conclusions of phyllode morphology and developmental divergence. The difficulty arose because the nature of the change in blade form masked the true change in proportion between petiole and blade, forcing investigators to rely on blade form as an indication of the length of the petiole. Underlying much of the erroneous judgement of phyllode morphology was the assumption that the developmental change proceeded by Goebel's metamorphosis concept. Actual comparative developmental analysis of serial appendages in seedlings of phyllodineous species of Acacia have not only negated the metamorphic concept but have underscored the necessity of truly comparative developmental data for a correct assessment of organ homologies. SATTLER, ROLF. Biology Department, McGill University, Montreal, Quebec H3A IBl - Homology - a continuing challenge. According to Sneath and Sokal (1973) morphological correspondence and common ancestry are the two basic ideas at the root of the many different homology concepts which have been proposed. Definitions of homology in terms of common ancestry have been criticized by many authors for a variety of reasons. The other idea of homology which refers to morphological correspondence may be based on a philosophy of essentialism. Accordingly, two structures are homologous whenever they are essentially similar. Remane (1952) specified three main criteria and a number of auxiliary criteria which determine what is meant by essential similarity or morphological correspondence. Although many structures can be easily homologized according to these criteria, difficulties arise when different criteria lead to contradictory honologizations. A resolution of such difficulties is achieved by distinguishing different kinds of homology. Other difficulties are resolved by the application of a semiquantitative or quantitative homology concept which leads to the recognition of partial homologies (or partial correspondences) in contrast to total homologies (1:1 correspondences). STEVENS, Peter F. Harvard University Herbaria, 22 Divinity Avenue, Cambridge, MA 02138 - Homology and plant systematics. I discuss aspects of similarity, homology, and the relationship between them. (1) The operations by which we decide that characters are similar (potent- ially homologous) and so usefui in phylogenetic analysis are little discussed in recent taxonomic literature; we face similar problems irrespective of our taxonomic school. These problems include the evaluation of conflicting criteria of similarity; the reification of terms which may then lead to untested assumptions about similarity; and the criteria used in the delimitation of character states. (2) The translation of assessments of simple similarity to hypotheses of homology, similarity as a result of common ancestry, are crucial. The translation involves cladistic analysis of all similar characters; those derived characters that are con- gruent with the best-supported cladogram may be hypothesized to be homologies; such hypotheses are tested both against new data and more detailed studies of the original similarities. In cladistic analysis, homology and synapomorphy are synonymous and neither can be partial. (3) The relationships between organisms, criteria for assessing similarity in structure of different organisms, and theories concerning the relationships between organisms that is decided by some estimate of similarity, are con- sidered. Emphasis is placed on the process of re- ciprocal illumination in an attempt to evaluate the claims of some morphologists and systematists as to the historical and/or epistemological priority of certain parts of the systematic process. TOMLINSON, PHILIP B. Harvard University, Harvard Forest, Petersham, MA 01366. - Homology: An empirical view. Comparative morphology in plants is presented as an empirical discipline into which "homology" (a set of theoretical concepts) is often inserted as an impedi- ment to analysis. Problems arise because homology is defined and homologous structures are established in a variety of ways. A priori rules applied in morphological analysis can result in circular reasoning. Observation can be relegated to a lower order of scientific endeavour, at the expense of theoretical analysis. Modular organisms, like the higher plants with extended heteroblastic ontogenetic sequences and indeterminate addition of parts, offer abundant op- portunity for misinterpretation, as compared with organisms with unitary construction. Their adaptive organization depends on a high degree of plasticity. Evolutionary development of plant form is a process of "fixing" this plasticity and regenerating it in novel ways. A particular confusion is that between "'structural similarity" which is a direct result of the organization of the individual, and is inherent
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