Until the 1930s biological science had not paid sufficient attention to the excretory function of plants. The interest in the ability of plants to release various compounds arose in connection with two discoveries. In 1930, the embryologist Boris Tokin reported that in plant excreta there are substances which he called "phytoncides", because they possessed an antimicrobial activity. Later, Hans Molisch (1937), based on numerous observations, concluded that chemicals released by plants can participate in a competition between plants in phytocenosis. At the same time the first monograph Die Stoffausscheidung der hoheren Pflanzen by Frey-Wyssling (1935) appeared, in which the excretory function of plants was considered, mainly from the anatomo-physiological viewpoint. Almost half a century after these fundamental works, the excretion by plants was studied in various fields of biology: plant anatomy and cytology, biocenol- ogy, plant immunology, and pharmacology. Anatomical and cytological data dealing with plant secretory structures are reviewed in detail in some monographs (Schnepf 1969; Vasilyev 1977; Fahn 1979; Buvat 1989; Denisova 1989).
Many publications are devoted to the role of plant excreta in phytocenosis, mainly in connection with allelopathic inter- actions. They are summarized in the books of Chernobrivenko (1956), Grodzin- skii (1965, 1991), Ivanov (1973), Rice (1974, 1984), Kolesnichenko (1976).