Introduction

Abstract

Parasitic weeds belong to a group of plants which have lost their autotrophic way of life during their developmental process (evolution). They are nutrition specialists to the disadvantage of their host plants. With special organs, the haustoria, they penetrate into the vessels of other plants in order to supply themselves with water and nutrients. Different developmental steps in the process of evolution can be understood by the existence of different degrees of parasitism. Facultative parasites like species within the genus Rhinanthus or Melamyprum represent those at the beginning of the process while obligate parasites represent more advanced plants. Obligate parasites are either hemi-parasites with chlorophyll containing species of the genus Striga and Alectra (Scrophulariaceae), or holoparasites without chlorophyll, e.g., of the genus Orobanche (Orobanchaceae). Instead of being root parasites like species of the genus Orobanche, Striga and Alectra, species of the genus Cuscuta (Cuscutaceae) are obligate shoot parasites.