Characteristics and modes of origin of weeds.
In synonymous types of Eupatorium micro-stemon, the weedy with 4 pairs of chromosomes, widespread in Central and South America, the nonweedy with 20 pairs, confined to uplands of Central America, the weedy characteristics were annual habit, quick flowering, photoperiodic neutrality, self compatibility, autogamy, economy of pollen and plasticity. The same characters were found in Ageratum conyzoides, a tetraploid (n = 20) which has spread all over the tropics, compared with A. macrocarpus (2n = 20), restricted to New World tropics. In Eupatorium hybrids the 4 large chromosomes form as many multivalents with the 20 small ones as their chiasma numbers will allow, and in the Ageratum spp. 10 of the chromosomes are homologous. In Oxalis pescaprae from South Africa, both (4n = 28) and (5n - 35) types have been carried elsewhere but only the pentaploid has spread widely by means of vegetative reproduction; heterosis, wide ecological tolerance and developmental homeostasis were probably the result of hybridization between a hexaploid and tetraploid. Many weeds, notably Eupatorium adenophorum[Eupatorium trapezoideum], show agamospermy.