STUDIES ON THE ONTOGENY OF THE DWARF MISTLETOES, ARCEUTHOBIUM. I. EMBRYOGENY AND HISTOGENESIS

COHEN, L. I. (Washington State U., Pullman.) Studies on the ontogeny of the dwarf mistletoes, Arceuthobium. I. Embryogeny and histogenesis. Amer. Jour. Bot. 50(4): 400-407. Illus. 1963.A complete developmental study on the origin and organization of the main tissue systems in the embryo of Arceuthobium reveals a greater degree of internal differentiation in the mature embryo than hitherto reported. The mature embryo consists of rudimentary cotyledons, a well-defined hypocotyl and a highly organized radicle. The latter develops from the remaining undifferentiated portion of the globular embryo soon after the embryonic cortex and procambium are differentiated. The radicular promeristem is organized into 4 initiating zones: (1) a uniseriate layer of apical surface initials; (2) a subapical zone of central initials; (3) a peripheral zone; and (4) a zone of procambium initials. Comparison between the dwarf mistletoe embryo and those of several nonparasitic angiosperms indicates that their ontogenies, in many respects, are essentially similar. THE GENUS Arceuthobium consists of a group of flowering plants which grow as parasites on various coniferous hosts. Owing to their many unique morphological features, these extraordinary plants provide a particularly interesting subject for study. Although some dwarf mistletoe structures have evolved along evolutionary lines similar to those of nonparasitic angiosperms, others are highly modified and extremely difficult to interpret in terms of classical morphology. While the aerial portion of the dwarf mistletoe plant clearly is an inflorescence, the homology of the haustorium has been the subject of considerable controversy, especially regarding the (uestion of whether or not the haustorium is homologous with a root (Coheni, 1954). Accordinig to anl affirmative view (Peirce, 1905), the haustorium is a special outgrowth of a true but modified radicle, an interpretation rejected by Heinricher (1915, 1923) and Von Tubeuf (1923). The latter 2 authors denied the existence of a radicle and maintained that the haustorium is a product of the hypocotyl. Heinricher (1923) described the spread of the haustorium as mycelium-like and the development of the absorption system as thalloid. Rootcap, pericycle, endodermis, typical phloem and the radial exarch xylem are structures generally present in roots I)ut are lacking in the haustorium of Arceuthobiumn. The primary basis for considering the haustorium I Received for publication July 2, 1962. This paper is a portion of a dissertation submitted in lartial fulfillment of the requirements for the degree of I)octor of Philosophy in the Department of Biology at the University of Oregon, Eugene. Sincere appreciation is extended to Dr. Sanford S. Tepfer for his helpful suggestions during the course of this study and to I)r. Noe Higinbotham for reading the manuscript. The help of D)r. M. Safeeulla with the photomicrographs and the use of certain photographic equipment and materials of the Department of Plant Pathology, Washington State University, are gratefully acknowledged. 2 Present address: Science Department, State University of New York, Cortland, New York. as a root is its function in anchorage and absorption. In view of its anomalous nature, Eichler (1868) and Goebel (1905) preferred to interpret the haustorium as an organ sui generis. Thoday and Johnson (1930) later coined the nonrestrictive term now in general use, i.e., the "endophytic system." In a recent report, Kuijt (1960) objected to the term "haustorium" on grounds that such usage is morphologically meaningless. The endophytic system, according to Kuijt, cannot be placed into any of the classical morphological categories such as shoot, rhizome, or thallus. To resolve this controversy, a complete developmental study from the earliest stages in embryogeny through the establishment of the endophytic system within the host seemed highly desirable. By studying the origin of the main tissue systems in the embryo and their subsequent development in the seedling, information was obtained regarding the role each particular tissue plays in establishing the endophytic system within the host. The present paper deals with the development of the embryo, including the origin of the primary meristematic tissues. A second paper, to be published in the following issue, will treat seedling development, mode of penetration and establishment of the haustorial system within the host as well as a general disc ussion on the homology of the endophytic system. MATERIALS AND METHODSThree species of dwarf mistletoe, collected in various stages of development, were studied during the course of this investigation: Arceuthobium americanum Nutt. ex Engelm., A. douglasii Engelm., and A. campylopodum Engelm. Of these species, A. americanum and A. campylopodum (f. campylopodum) were collected at weekly intervals during March-November, 1959 and 1960, mostly in the area along the east shore of Crescent Lake, east of Willamette Pass, in Oregon. Arceuthobium cam pylopodum f. laricis and A. douglasii were