THE ANATOMY OF THE ENDOPHYTIC SYSTEM OF THE DWARF MISTLETOE, ARCEUTHOBIUM CAMPYLOPODUM

Abstract

IN THE GENUS Arceuthobium, as in all the Loranthaceae, the morphology of the system has been a subject of controversy. Studies have been restricted to a few species and reports have been made without apparent correlation of the data available in the literature. Although Arceuthobium campylopodurm has received some previous attention, the study of its form and anatomy provides information which can be correlated with the data available for other species. Peirce (1905), reporting on work with A. occidentale (A. campylopodum Engelm. forma campylopodum), states that the primary haustorium of Arceuthobium is evidently the primary root of the seedlings and the haustorium itself a special outgrowth of the tip of the radicle. He considered the haustoria to be modified roots which penetrate the medullary rays and form direct connections with the tracheids of the host. Eichler (1868) and Goebel (1905) interpreted the haustoria as organs sui generis. Solms-Laubach (1867), reporting on the studies of Arceuthobium oxycedri (DC.) M. Bieb., describes two types of strands based on size and position in the host tissue. The large strands, occupying the host bark, were called Rindenwurzeln and those occupying the host xylem were designated as Senkern. The Rindenwurzeln were described as circular in crosssection, consisting of a sheath of parenchyma cells surrounding a central zone in which tracheids are present. Two types of sinkers were recognized: the broad sinker located in the xylem ray, and the narrow sinker located in the xylem but not associated with the xylary ray of the host. Heinricher (1923), who worked with A. oxycedri, described the spread of the haustorium as mycelium-like and development of the absorption system as thalloid. He considered roots to be lacking in Arceuthobium and the absorptive system as the product of the hypocotyl. Like Peirce, he reported that the sinkers penetrated the xylem of the host, but generally his work substantiated the work of Solms-Laubach and contradicted many of Peirce's findings. Thoday and Johnson (1930), working with thie small, short-lived Arceuthobium pusillum Peck, coined the term endophytic system, a term which avoids a committment as to the nature of the haustorium. They described many strong strands of parasite tissue running in towards the wood and penetrating it at the medullary rays. Dufrenoy (1936), discussing the parasitism of A. campylopodum forma campy-