“ They are herbaceous plants growing from an underground much-branched rootstock from which spring slender aerial shoots which are green, ribbed, and bear at each node a whorl of leaves reduced to a toothed sheath. ”

“ In most species the fertile and sterile shoots are alike, both being green and leaf-bearing, but in a few species the fertile are more or less different, e.g. in E. arvense the fertile shoots appear first, in the spring, and are unbranched and not green. Any portion of the underground rhizome when broken off is capable of producing a new plant; hence the difficulty of eradicating them when once established. There are 24 known species of the genus which is universally distributed. ”