Studies on the Ontogeny of the Dwarf Mistletoes, Arceuthobium. Ii. Homology of the Endophytic System

Abstract

Cohen, L. I. (Washington State U., Pullman.) Studies on the ontogeny of the dwarf mistletoes, Arceuthobium. H. Homology of the endophytic system. Amor. 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 until the root apex is confronted by a spur shoot or by a fragment of raised bark. The root apex becomes attached to the host 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.

Penetration of western hemlock,Tsuga heterophylla, by the dwarf mistletoeArceuthobium tsugense, and development of the parasite cortical system

“…Once in contact with phellem, the penetrating structure ramified into filaments. Although surface initials at the leading edge of the penetrating structure participated in penetration, the penetration process was more than just surface initials initiating infection, as reported by Cohen (1963), or the radicle promeristem apparently growing into bark fissures as reported by Tainter (1971). Formation of the penetrating structure and its method of penetration is almost identical to that reported for needles of Abies spp.…”

“…However, only the existence of mechanical forces that crush host cells was demonstrated in dwarf mistletoe penetration (Kuijt 1960, Cohen 1963, Scharpf & Parmeter 1967). Parmeter and Platt (1968) demonstrated that A. abiet-…”

“…In some species, such as in Pinus, these obstructions are often flaps of bark tissue (Cohen 1963) and in others, such as Abies, they are often needle bases (Scharpf & Parmeter 1967) or possibly branchlet axils (Parmeter & Platt 1968). In the former case, surface initials may grow from the holdfast into one or several bark crevices, while in the latter case, an organized penetrating wedge forms within the holdfast.…”

Physiological Aspects of Parasitism in Mistletoes (Arceuthobium and Phoradendron). I. The Carbohydrate Nutrition of Mistletoe

“…This may indicate a change in the mechanism of transport possibly related to the absence of (5,6,16,28). When mistletoe shoots were girdled by exposing a portion of the stem to steam, host photosynthate failed to move through the injured tissue and the shoots subsequently died.…”

“…Injury caused by dwarf mistletoe is manifested in a variety of ways: reduction in tree growth, distortion of growth, recluction in wood quality, predisposition of trees to secondary attack, and reduction in seed yield seems less direct in Phoradendron (4,34). Differentiated phloem tissue has never been observed in Arceuthobium (5,6,16,28) and in Phoradendron it is present only in the aerial shoots (34). There appears to be no specialized relationship between the phloem of the host and the endophytic system of either Arceuthobium ( 5,9,22) or Phoradendron (4,34).…”