Importance of Leaf Distribution in Plant Identification

by Arieh Tal, 2015-16

Many of us are familiar with three often-used and important botanical terms for describing how leaves are arranged on a stem or branch: alternate, opposite and whorled. These terms can be crucial for enabling us to distinguish between different species. But, when presented with a large group of similar species, such as the goldenrods, all of which have the same kind of leaf arrangement, we may need to consider other characteristics. The leaves of all goldenrod species in New York are alternately arranged on stems and branches, so we won't get much mileage out of that characteristic.

However, if we consider the manner in which leaves are distributed vertically on a plant, from base to summit, we may be more successful. Are most or all of a plant's leaves at the time of flowering located at the base of the plant, as in a basal rosette, or are they primarily distributed vertically along the stem, either as well-developed leaves or just greatly reduced, scale-like leaves?

Let's consider a few examples. The common dandelion (Taraxacum officinale) has no stem leaves. The smaller pussytoes (Antennaria howellii) has well-developed basal leaves, but only very small, scale-like stem leaves. Other species, like tall goldenrod (Solidago altissima) maintain only well-developed stem leaves at flowering, and quickly lose their basal leaves early in the season. And in between these types, we have many species which maintain both basal and stem leaves at flowering, such as early goldenrod (Solidago juncea).

Gleason and Cronquist (1991) often used the terms basally-disposed (i.e., oriented toward the base) and chiefly cauline (i.e., mainly on the stem) in their descriptions of species of goldenrods and asters. Basally disposed refers to a pattern of leaf distribution, in which a plant's largest leaves are the basal leaves, and the mid- to upper-stem leaves (if present) become progressively smaller toward the summit of the stem. When leaves are basally disposed, leaf shape may also vary by position on the stem. For example, stem leaves may become progressively narrower and/or less prominently lobed or toothed toward the top of the stem.

Solidago juncea maintains leaves that are basally disposed, often with a basal rosette and well-developed stem leaves that become progressively smaller, narrower and less prominently toothed upward along the stem. By comparison, Solidago rugosa has leaves that are chiefly cauline, and its stem leaves gradually, or not at all, become smaller and less prominently toothed toward the inflorescence.
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                                     Figure 1: (L) Leaves basally disposed (S. juncea); (R) Leaves mainly on stem (S. rugosa).
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Some goldenrod species similar to S. juncea (basally-disposed) include: S. arguta, S. nemoralis, S. patula, S. bicolor, S. squarrosa and S. uliginosa. Goldenrod species similar to S. altissima (chiefly cauline), include: S. rugosa, S. canadensis, S. gigantea and S. ulmifolia.

However, it gets more interesting than this. The vertical distribution a plant's leaves affects its ability to compete with other plants in its vicinity, and thus in the types of habitats in which it can succeed. Plants with leaves that are basally disposed (or entirely basal), cannot compete well with tall plants that maintain only well-developed leaves at flowering. That's why we find Solidago juncea and Antennaria neglecta in drier, more open and/or disturbed habitats with relatively sparse vegetation. You won't find them in the midst of a damp thicket full of stout bullies, such as Solidago altissima. Though the latter can often be found in dry, open places, it doesn't do quite as well there, nor does it spread as aggressively in such places.

There are exceptions to every rule, and some of the pussytoes that often grow on lawns are exceptions - sort of. The common Antennaria neglecta is typically found on lawns in spring, sometimes forming colonies from stolons. This species has basally-disposed leaves with only one or two, if any, very diminished, stem leaves. And it grows nicely with typical turf-forming lawn grasses. It's able to succeed, ironically, thanks to the mowing, which serves as a great equalizer. For this species, stolons work just fine.
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                                                    Figure 2: Leaves basally-disposed, with stolons. Antennaria neglecta
 
 

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