Anatomy of the Endophytic System of the Mistletoe, Phoradendron flavescens

Abstract

The tissues comprising the cortical haustoria and sinkers in the endophytic system of Phoradendron flavescens were studied in plants parasitizing the walnut, Juglans Hindsii. Cortical haustoria-longitudinally oriented strands in the host bark-have a central cylinder containing xylem and phloem inclosed by a parenchymatous cortex. The xylem consists of tracheary elements and parenchyma cells. The phloem consists of sieve-tube members, companion cells, and parenchyma cells. Cortical haustoria having secondary growth contain, in addition, fibers in their secondary xylem. Sinkers-radially oriented extensions into the host wood-occur at intervals along a cortical haustorium. Each normal sinker consists of three distinct parts: (1) a parenchymatous part imbedded in the host phloem; (2) a meristem corresponding in position to the host cambium; (3) a part imbedded in host wood and composed of two kinds of cells, parenchyma cells and tracheary elements, both more or less isodiametric. Additionally, many sinkers have a centrally located, radially oriented strand of vascular tissue. The xylem of this strand extends from the cortical haustorium to near the end of the sinker. The phloem, which contains sieve elements, accompanies the xylem for a short distance; it does not extend to the meristematic region. The tracheary elements of the radially oriented strand are connected with the tracheary elements of the sinker part, which is imbedded in the host wood. The parasite elements that occur at the periphery of the sinker are in contact with tracheary elements of the host, and pits and perforations may occur between them. Thus a continuous conduit exists, connecting tracheary elements in the host with those in the cortical haustorium of the parasite. During growth at their meristems some sinkers become progressively smaller (narrower or lower or both). Such sinkers may be displaced from the cambial cylinder; thus their part within the host wood becomes separated from the continuous endophytic system. Inclusion of isolated sinker parts within the host wood and certain other phenomena suggest competition between parasite and host for space in the cambial cylinder. In the part of a sinker imbedded within the host wood, thick accumulations commonly occur in parenchyma cells on walls that are in contact with tracheary elements of the host or the parasite.