Adventive weeds with particular reference to their significance in Canada.

The general ecological characteristics of weeds at the individual and population levels were evaluated and assessed from the available literature. Adventive weed species were found to be difficult to define as a class, but were worth studying for their economic significance. The most adequate definitions of such weeds mention their characteristic aggressiveness and their tendency to occupy disturbed sites. Weeds tend to have inbreeding reproductive systems, but some species form inter-generic crosses, resulting in genetically complex populations. Adventive weeds were found to be different from non-weeds in a number of respects, being more tolerant of a range of environmental conditions and having more effective long distance dispersal mechanisms. Weeds tend to have general purpose genotypes and plasticity in their phenotypic expression. Before man's alteration of much of the earth's surface, weeds were probably confined to naturally disturbed sites. Since the development of agriculture, man has created the most important conditions influencing weed reproduction and survival. The use of herbicides is a most important current aspect of man-weed interactions. The characteristics and distribution patterns of adventive weeds in Canada were analysed from the available government sponsored weed survey reports. The Canadian weed population was found to include a high proportion of species of Eurasian origin. The proportion of perennial species was found to be similar to the proportion of annuals and biennials taken together. There were far fewer annuals than biennials. Many individual weed species had a distribution which covered all of southern Canada. British Columbia, and the souther~ parts of Ontario, Quebec and the Maritime provinces have a richer weed flora, and a higher proportion of alien species than that found in the rest of Canada. Physical and historical reasons for this distribution are discussed.