STUDIES ON THE ONTOGENY OF THE DWARF MISTLETOES, ARCEUTHOBIUM. II. HOMOLOGY OF THE ENDOPHYTIC SYSTEM

COHEN, L. I. (Washington State U., Pullman.) Studies on the ontogeny of the dwarf mistletoes, Arceuthobium. II. Homology of the endophytic system. Amer. Jour. Bot. 50(5): 409-417. Illus. 1963.-Development of the seedling in Arceuthobium, as well as the mode of penetration and infection, is described. The promeristem of the root apex of the seedling, as in the embryo, is composed of 4 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 procambial initials. After germination, the seedling grows on the host surface unitil the root apex is confronted by a spur shoot or by a fragment of raised bark. The root apex becomes attached to the ho3t by means of a massive holdfast. It is the procambial initials which actually invade the host tissue. After penetrating the host, the individual procambium initials proliferate in the cortex, each giving rise to a cortical strand. It was concluded that the endophytic system of Arceuthobium cannot be interpreted in terms of classical concepts of plant homology; it is, in fact, an organ sui generis. THE ORIGIN and organization of the main tissue systems in the embryo of Arceuthobium were described in the first paper of this series (Cohen, 1963). A comparison between the dwarf mistletoe embryo and those of nonparasitic angiosperms indicates that their ontogenies are essentially similar. Although the anatomy of the seedling in Arceuthobium has been described (Kuijt, 1960), details of its development are fragmentary and imperfectly known. Knowledge of precisely how penetration occurs is completely lackingf (Peirce, 1905; Kuijt, 1960). An interesting controversy has revolved around the correct morphological interpretation of the extensive haustorium within the host, the endophytic system. The latter has been called "roots" (Peirce, 1905; Heil, 1923), "rhizomes" (Von Schrenk, 1900) and "thallus" (Heinricher, 1923). Peirce (1905) considered the endophytic system to be an outgrowth of a true but modified radicle; Heinricher (1915, 1923) and Von Tubeuf (1923), on the other hand, declared it to be the product of the hypocotyl. Eichler (1868) and Goebel (1905) held that the endophytic system in Arceuthobium is an organ sui generis. Other workers, although ' Received for publication July 2, 1962. This paper is a portion of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Biology at the University of Oregon, Eugene. Sincere appreciation is extended to Dr. Sanford S. Tepfer (U. of Oregon) for his helpful suggestions during the course of this study and to Dr. Noe Higinbotham (Washington State University) for reading the manuscript. The help of Dr. 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. leaning towards the sui generis idea, have avoided making a definite commitment as to the nature of the haustorium (Thoday and Johnson, 1930; Kuijt, 1960). The present paper deals with seedling development, mode of penetration and establishment of the endophytic system within the host. On the basis of this study, the homology of the endophytic system is discussed and it is concluded that the endophytic system is a new organ. The materials and methods employed in this study are essentially the same as those cited in the first paper in this series. THE SEEDLING-The radicular promeristem of the embryo with its compactly a-ranged cells consists of 4 zones: (1) a unicellular layer of surface initials; (2) a subapical zone of central initials; (3) a peripheral zone; and (4) a zone of procambial initials (Cohen, 1963). Each zone can be identified by a characteristic pattern of cell divisions; all cells derived from the radicle are interpreted to constitute the root of the seedling. A typical seedling is illustrated in Fig. 1. The elastic cap of endosperm tissue ruptured by the growing root apex still persists as a thin sheath-like collar around the seedling. During the early stages of germination, the compact cellular arrangement of the embrvonic radicle undergoes a gradual change to a type of organization in which the cells assume a loose spatial arrangement (Fig. 2). Intercellular spaces within the root apex of the seedling become quite large and the individual cells tend to become spherical. The superficial layer and sometimes the subjacent layer of promeristem cells remain unaltered in appearance. The loosely organized cells of the root promeristem of the seedling are essentially similar to those of the compactly organized cells of the embryo especially with [The Journal for April (50: 301-407) was issued April 29, 1963] AMERICAN JOURNAL OF BOTANY, Vol. 50: No. 5, 1963