A Survey of Weed Leaf Stomata and Trichomes

Thirty-five species of weeds were grown in a field nursery in order to obtain data on the number, size, and distribution of their stomata. Mature leaves were cleared, stained, and mounted for microscopic examination.The mean numbers of stomata per mm2 of leaf surface ranged from 2.3 to 315.2. The mean lengths of the guard cells varied from 21.6 to 57.0 μ. Twenty species had significantly more stomata/mm2 on the lower than on the upper surface. Only three species had significantly more on the upper surface. Between species, there was negative correlation of stomatal number with guard cell length, but between surfaces within species the lengths remained fairly constant despite differences in numbers. Plants grown in partial shade had slightly more stomata/mm2 than those grown in full sun. The same was true of leaves sampled in July compared with those sampled in June. In neither case could the differences be attributed solely to environmental factors.A series of drawings was made depicting the different stomatal characteristics and types of trichomes. It was estimated that the leaves of nine species had enough pubescence to influence retention or penetration of sprays.