THE DESTRUCTION OF XYLEM BY A PLASMODIAL PARASITE IN SEEDLINGS OF PISUM SATIVUM

The seedlings of Pisum sativum var. ‘Alaska’ grown either in complete darkness or in partial red light are often afflicted by a plasmodial parasite, which either partially or completely destroys the tracheids. The vegetative stage of the organism consists of small cells connected with each other by hyaline filaments. These cells are seen inside the tracheids, where they presumably hydrolyze cellulose to separate the lignified helical wall thickenings from the cell wall. The spirals are then incorporated into a plasmodium. The plasmodia are found both inside and outside the plant tissue. The younger portions of the seedlings often contain large plasmodia with engulfed helical wall thickenings, which are gradually dissolved, presumably by means of a peetinolytic enzyme, at the time of sporulation. The spores (4–5μ) are borne in small numbers, ca. 10–16 per well‐defined oval or elongate sporangium. The pitted vessels are not destroyed by the organism. Pea seedlings can be badly infected without showing externally visible pathology. Unless the xylem is destroyed and the plant tissues are filled with plasmodia, recovery from the infection is possible. Pisum sativum, a homozygous variety ‘L’ grown aseptically in light on an agar medium, also contained all the stages in the development of this organism.