Two Species of Yellow Wood-sorrel

Two species of wood-sorrel (Oxalis) with yellow flowers grow in my neighborhood. The most common is Yellow Wood-sorrel Oxalis stricta which has bright green foliage and grows relatively erect. The other species seems to be Creeping Wood-sorrel Oxalis corniculata. This plant has a distinct purplish cast to its foliage and has a lower growth habit. Oxalis corniculata also appears to be more densely hairy, especially the fruits (pods). Although Weeds of the Northeast (Uva et al., 1997) say that leaf color is variable in both species and cannot be used as a diagnostic characteristic in my experience wood-sorrel plants with low growth habit always seem to have a purplish cast to the foliage. Gleason and Cronquist also mention this feature under Oxalis corniculata but not under O. stricta, so perhaps the purplish foliage color is diagnostic for Creeping Wood-sorrel O. corniculata in spite of Uva et al.

A Tree Grows in Brooklyn

The tree that grows in Brooklyn immortalized by Betty Smith is, of course, Ailanthus altissima, the Tree of Heaven. This is the urban tree par excellence, able to take root, grow and thrive where few other trees can. In spite of its 'weed' status it is not an unattractive tree.

The compound leaves consist of leaflets that typically have a pair of large basal teeth with glandular openings on the underside.

Books for Identifying Plants

The book that introduced me to North American plants and which I still find quite useful is North American Wildlife edited by Susan J. Wernert (1982; Reader's Digest Association, Pleasantville, New York), which is probably the best one-volume guide to American animals, plants, algae and fungi. Plants are covered in two sections in this book: Trees and Shrubs (pages 286-331) and Wildflowers (pages 332-507). Unusually for a non-technical work the plants are arranged in systematic order, the wildflowers starting with the Saururaceae and ending with the Orchidaceae. The family names are not actually indicated in the book so I remedied this in my copy by inserting the names at the top of the pages. Although many field guides to flowering plants are organized according to flower color and may therefore appear to be easy to use such artificial classifications make it difficult to recognize family (or generic) characteristics. I don't think you can 'learn' your plants in any meaningful way without some appreciation of their taxonomic affinities, so I find the treatment of wildflowers in the Reader's Digest book useful. Two field guides that are organized according to flower color which I use often are the old classic A Field Guide to Wildflowers: Northeastern and North-central North America by Roger Tory Peterson and Margaret McKenny (1968; Houghton Mifflin Company, Boston) and the more recent Wildflowers in the Field and Forest: A Field Guide to the Northeastern United States by Steven Clemants and Carol Gracie (2006; Oxford University Press, New York). The illustrations in the Peterson guide are drawings (including some in color), while those in the Clemants and Gracie book are photographs. To these titles must be added two excellent books on weedy spontaneously growing plants: Weeds of the Northeast by Uva et al. (1997) and Wild Urban Plants of the Northeast by Del Tredici (2010), which I have mentioned in a previous post; they are both very useful for getting to know the flora of New York City. Grasses and related plants are usually not included in field guides to flowering plants but one slim volume with excellent line drawings that I have found useful for identifying them is Field Guide to Grasses, Sedges and Rushes of the United States by Edward Knobel (revised by Mildred E. Faust, 1977; Dover Publications, New York). For trees, my preferred field guide is Trees of North America: A Guide to Field Identification by C. Frank Brockman (2001; St. Martin's Press, New York), a copy of the original edition of 1968 of which I picked up many years ago at the Strand Bookstore. Another useful book for identifying trees which is profusely illustrated with black and white photographs of leaves, flowers, fruits, bark and twigs is The Tree Identification Book by George W. W. Symonds (1958; HarperCollins, New York). For most city trees the handy guide New York City Trees: A Field Guide to the Metropolitan Area by Edward Sibley Barnard (2002; Columbia University Press, New York) is convenient to use. Though there are many excellent field guides to wildflowers and trees (with new ones appearing regularly) books useful for identifying shrubs and vines are not very numerous. One book that I have found useful is The Shrub Identification Book by George W. D. Symonds (1963; HarperCollins, NewYork), which is a companion volume to the author's tree book and is similarly illustrated with numerous black and white photographs. It covers those many species of plants that tend not to be included in popular plant field guides because they don't fall neatly into the 'wildflowers' or 'trees' categories. This list of books includes only titles that I own or have ready access to and which I use regularly. One wildflower guide which I don't own but which was once recommended to me by a professional botanist is Newcomb's Wildflower Guide by Lawrence Newcomb (189; Little, Brown and Company, Boston). Lastly, one should mention the 'ultimate authority' when it comes to identifying plants in the northeastern United States: Manual of Vascular Plants of Northeastern United States and Adjacent Canada (Second Edition) by Henry A. Gleason and Arthur Cronquist (1991, 2004 or later reprint; The New York Botanical Gardens Press, New York). The absence of illustrations in Gleason and Conquist is remedied in the slightly larger format companion volume Illustrated Companion to Gleason and Cronquist's Manual: Illustrations of the Vascular Plants of Northeastern United States and Adjacent Canada by Noel H. Holmgren (1998; The New York Botanical Gardens Press). Gleason and Cronquist's massive tome is definitely not for the casual reader but for the serious naturalist it provides answers that other publications cannot. 'Gleason and Cronquist' covers clubmosses, horsetails, ferns and relatives, in addition to flowering plants, i.e. all vascular plants.

Chicory in Bloom

Chicory grows abundantly alongside roads and highways and in road median strips. This photograph was taken on June 20th beside Flatbush Avenue, at the Flatbush Avenue/Utica Avenue/Avenue S intersection. Excellent descriptions and photographs of the parts of a Chicory plant can be found here at the Missouri flora website.

Close-up of Chicory Flowerhead

This close-up of a Chicory (Cichorium intybus) flowerhead shows individual ligulate (ray) flowers with dark blue anthers around the style and bifurcate stigma.

Spotted Spurge

Spotted Spurge (Euphorbia maculata) is another common plant of sidewalk cracks and other urban habitats. Though superficially similar to Prostrate Knotweed in its growth form it is easily distinguished from the knotweed by its opposite (not alternate) leaves, reddish-brown stem and the milky sap that oozes from broken stems. A variable feature is the elongated dark maroon blotch on the leaves. In some plants - such as this one - the maroon mark is lacking entirely.

Prostrate Knotweed

The knotweed (Polygonum) with tiny white flowers at its nodes (joints) found growing in a range of different habitats, including disturbed and heavily trampled areas - such as cracks in city sidewalks - is treated as a single variable species, Prostrate Knotweed (Polygonum aviculare), in many plant books. However, according to some authors several distinct species are involved and the species that occurs in backyards and sidewalk cracks is the Dooryard Knotweed (Polygonum arenastrum). One important distinction between true P. aviculare and P. arenastrum (and related species) is variation in leaf size. Plants that have axillary flowers with more or less same-sized leaves are said to be homophyllous, whereas plants with seemingly terminal inflorescences with the flowers actually axillary to much reduced leaves or bracts are said to be heterophyllous. P. aviculare is described as heterophyllous while P. arenastrum is regarded as homophyllous. All of the Prostrate Knotweed type plants I have examined appear to be homophyllous.