STUDIES ON THE ONTOGENY OF THE DWARF MISTLETOES, ARCEUTHOBIUM. III. DEVELOPMENT OF THE INFLORESCENCE

In vascular plants, the apical meristem of the shoot normally represents a continuation of growth in the apical meristem of the embryo itself. This is not the case in Arceuthobium. Here the shoot apex of the embryo is rudimentary and eventually dies after infection of the host occurs. The inflorescence of Arceuthobium is, therefore, an adventitious structure originating in the endophytic system rather than from the shoot apex of the seedling. Inflorescence buds arise in either of 2 ways. In some species (A. douglasii and A. americanum), buds first appear as small meristematic protuberances on the outer surface of cortical strands. In other species (A. campylopodum), the buds arise at the ends of short branches. The former, or diffuse, type gives rise to inflorescences along the entire surface of the host branch; in the latter, or condensed, type infloreseences are formed in clusters. Early ontogenv of the inflorescence apex of both types is described. Studies of subseqtient growth of the inflorescence apex show 5 well-defined plastochronic stages: (1) maximal area stage; (2) minimal area stage; (3) early post-minimal stage; (4) late post-minimal stage; and (5) pre-maximal stage. THE DWARF nmistletoe plant consists of 2 morphologically distinct parts: an extensive haustorial system within the host, and an aerial portion (the inflorescence) of spike-like cymes which bear either staminate or pistillate flowers. Whereas the anatoin-y of the haustorial system of several species of dwarf mistletoes has already been described in considerable detail (Solms-Laubach, 1867; Heinricher, 1923; Thoday and Johnson, 1930; Cohen, 1954; Kuijt, 1960; Srivastava and Esau, 1961; and others), details of inflorescence development are fragmentary and imperfectly known (Thoday and Johnson. 1930; Cohen, 1954; Kuijt, 1960). In vascular plants, the shoot apical meristem normally represents a continuation of growth in the apical meristem of the embryo itself. This is not the case in Arceuthobiurn. Here the shoot apex of the embryo is rudinmentary; it eventually dies and decays along with the remaining external portion of the seedling after the host is infected (Cohen, 1963b). The inflorescence in Arceuthobium is therefore an adventitious structure which originates in the endophytic system located within the host plant rather than from the shoot apex of the seedling. The present investigation was designed to study inflorescence initiation and development and to provide bases for comparing inflorescence development in Arceuthobium with that of several nonparasitic angiosperms. From this study, it is concluded that the aerial portion of the plant (the inflorescence) has evolved along evolutionary lines similar to those of angiosperms with an autotrophic mode of nutrition. The materials and methods employed in this study are essentially I Received for publication August 7, 1964. the same as those cited in previous papers in this series (Cohen, 1963a,b). INITIATION OF THE INFLORESCENCE-Buds which give rise to inflorescences (the aerial shoots) are formed in the peripheral region of the endophytic system (Thoday and Johnson, 1930; Cohen, 1954). They first appear either as small meristematic protuberances on the outer surface of cortical strands or at the ends of short branches. In some species of Arceuthobium, buds are distributed along the entire length of the strand; in other species, the buds are localized in clusters which are joined at their bases in a large cortical plexus. The endophytic system of A. douglasii Engelm. and A. americanum Nutt. ex Engelnm. are examples of the diffuse type. Penetration of new host tissue by these mistletoes is lengthwise and growth of the endophytic system keeps pace with that of the apices of the host. Kuijt (1960) refers to this type of endophytic growth as being isophasic (in contrast to the anisophasic growth shown by the condensed type). The systemiiic infection in the diffuse type is extensive but the resulting hypertrophy is not pronounced externally. New buds are formed acropetally, and it is possible by means of serial sections to obtain a complete series of developmental stages. In studying my material, 2 difficulties were encountered: the first was to distinguish some of the earliest bud prinmordial stages from contiguous cortical strand tissue; the second involved finding properly oriented older buds still embedded within the cortex of the host. Sectioning of perfectly oriented bud material was, therefore, a matter of chance. In the condensed system, represented by A. campylopodum Engelm., clusters of buds are formed radiating from a major center of infection