Developments during the first ten minutes of geotropic stimulation in plant seedling shoots are reviewed. Topics include induction and curvature; early processes; the relationship between auxin, electric field, calcium, and differential growth; gravity reception leading to Went-Cholodny transport; and comparison of root and shoot. Early processes reviewed are sedimentation of amyloplasts, release of ethylene, rise of electrical and auxin asymmetry, redistribution of calcium, asymmetric vascular transport, increase in tendency to deposit callose, and simulation of putative exocytotic voltage transients.
Virtually all scenarios for the long-term habitation of spacecraft and other extraterrestrial structures involve plants as important parts of the contained environment that would support humans. Recent experiments have identified several effects of spaceflight on plants that will need to be more fully understood before plant-based life support can become a reality. The International Space Station (ISS) is the focus for the newest phase of space-based research, which should solve some of the mysteries of how spaceflight affects plant growth. Research carried out on the ISS and in the proposed terrestrial facility for Advanced Life Support testing will bring the requirements for establishing extraterrestrial plant-based life support systems into clearer focus.
The ancient Greeks recognized abscission as one of the important botanical phenomena; the writings of Theophrastus (285 B.C.) include leaf abscission, particularly the differences found among deciduous and evergreen plants and the effects of environmental factors. As the subject is conceived today it concerns the shedding of any part of the plant.
Naturally-occurring auxins, of which indoleacetic acid is the best known example, have a striking apex-to-base polar transport. This may have led to the belief that all plant regulators have a rather specific mode of absorption and of transport. However, since plant regulators now (5) include a wide variety of organic compounds this seems most unlikely.
Many of the phenomena which are involved in the absorption and translocation of regulators in plants are well known. However, a unified picture seems lacking. It is a rather new field, which one will not find discussed in standard texts of plant physiology. The significance of this subject is more than academic. In recent years sprays have been applied to plants for a multitude of practical purposes (106a).
New techniques have helped in the understanding of the principles of absorption and translocation. Enzymatic digestion is used for the isolation of the plant cuticle, a principal barrier through which materials must pass. Electron microscopy is showing its detailed structure. Radioactive tracers and chromatography have been useful in the microanalysis of materials, revealing their distribution on and in the plant. The author has attempted to outline the broad principles involved.
this area one frequently proceeds without knowledge of the vast already existing experimental material which must be adequately evaluated before it may be described as real progress. Conway (8) has treated the general problem of a "redox pump" for ions from a thermodynamic point of view, but his scheme shows no salient features which may be distinguished from the theory of anion respiration. The author presented a review of the anion respiration to the symposium on active transport of ions in Bangor, England in 1952 (12 ). The substance of the present article is much the same, but the discussion of the more impor tant literature has been extended.
Naturall y occurring chelates of heavy metals are probably important sources of these elements in the soil and undoubtedly this is the primary form in which they are absorbed and translocated in plants. The use of synthetic chelates as nutrient sources for agricultural plants has developed within the past 10 years. Following the development of practical applica tions for synthetic che1ates in agriculture in 1952, research has been stimu lated toward synthesizing new chelates and studying their properties. Be cause these new compounds can be made with a wide range of properties they are being used not only as agricultural sources of heav y metals but more important as tools to advance our knowledge about mechanisms of up take and translocation of these elements in plants. Wallace (1) reported in 1956 that about 200 papers had been written concerning the use of che lates in soil and plant nutrition. Since that time several hundred other stud ies have been published. This doe s not include the thous ands of papers written on the analytical uses of chelates, their synthesis, chemical prop erties and industrial Uses. A number of books have been written that COver various phases of chelation and the readers are referred to these for defi nitions and a broader scope of the subject (2 to 5). Many of the papers written on chelates in plant nutrition are contro versial. In some cases, apparently correct observations were made but lack of general information precluded drawing conclusions. In others, lack of replication and thoroughness of study has undoubtedly caused misinterpre "tation of observations. Only a relatively few of the published papers will be cited in this review. However, an effort will be made to COver what this reviewer believes to be the pertinent