Distinguishing Lactuca Species

 

Lactuca is a genus in the Composite family of plants (Asteraceae).  Three species are commonly found in the New York - New England region:  L. biennis, L. canadensis and L. serriolaL. hirsuta and L. sativa have also been documented for the 7-state region, but are rare or highly uncommon in the wild.  Varieties of L. sativa are the source for cultivated, culinary lettuces.  Pictures and a more complete description of L. hirsuta can be found at the website:  Illinois Wildflowers.

Our species are similar in several ways, and can sometimes be difficult to distinguish.  They're taprooted annuals or biennials, with milky sap.  They all have relatively small, ligulate flower heads (with approx. 12-50 florets), with either pale blue or yellow corollas.  They tend to have stems that are hairy or bristly below, but become glabrous and glaucous above and within the inflorescence.  Leaves are usually glabrous on the upper surface and either pubescent or bristly on the lower surface midvein.  Stem leaves are alternate for all species.  Basal and stem leaves are often lobed, with toothed or prickly margins.  Leaves within the inflorescence are greatly reduced in size or reduced to minute bracts.  Involucres of fresh heads are cylindrical in shape, but become evidently swollen at the base in fruit.  The fruits (cypselae) are typically flattened, with prominent veins (ribs) and usually have an apical beak, short and stout in L. biennis, but long and thin in L. canadensis and L. serriola.

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Lactuca biennis:  Our tallest species, which can reach as much as 8 feet in height, has pale blue (varying to whitish) florets, and an overall inflorescence that tends to be taller than broad and congested.  Primary leaves have lobes that are typically fairly broad, and sinuses that are narrow and deep.  Cypselae are dark brown to black and flattened, with numerous  evident veins on both faces, and an extremely short beak.  Pappus bristles are light brown to grayish.
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Lactuca canadensis:  Can also be quite tall, reaching about 6 feet in height.  It has bright yellow florets, varying to yellow-orange, and an overall inflorescence that tends to be taller than broad, but somewhat less congested than that of L. biennis.  Lower primary leaves have narrow lobes and sinuses that are wide; but upper leaves may become narrowly lanceolate and unlobed (i.e., entire).  Cypselae are dark brown to black and flattened, with 1-3 evident veins on both faces, and a long, narrow beak.  Pappus bristles are white.
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There is another form of this species, occasionally encountered, with similar inflorescences and florets, but leaves that are unlobed, with finely toothed (denticulate) margins.  Contemporary taxonomists tend not to recognize varieties of L. canadensis, but this form may correspond to "var. obovata" as mentioned by Fernald (1950).
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                   Alternate form of Lactuca canadensis.  Left:  unlobed stem leaves.  Right:  overall inflorescence.
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Lactuca serriola:  Reaching about 5 feet in height, it's not quite as tall as the other two species.  It has pale yellow florets and an overall inflorescence that is often flat- or round-topped, as tall as broad.  The primary leaves are usually broadly lobed, with shallow, wide sinuses, and finely toothed (denticulate) margins.  Midveins on leaf lower surfaces are conspicuously prickly.  Leaf bases of this species are distinctly auriculate clasping and "twisted", causing the leaves to point outwards (spreading) or upwards (erect).  Cypselae are light brown to grayish, flattened, with numerous evident veins on both faces, and a very long, thin beak.  Pappus bristles are white.
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Comparison with Sonchus species:  Species in the genus Sonchus are closely related to and morphologically similar to Lactuca species in several ways.  They all have alternate leaves, milky sap, ligulate flower heads, and involucres that are conspicuously swollen basally, post-anthesis.  They also all mature fruit quickly, and are often in flower and fruit at the same time.  Sonchus species tend to be shorter in height than Lactuca species, but I've seen the occasional Sonchus reaching about 5 feet (usually as garden weeds).  Some Sonchus species may have lobed leaves, at least on some specimens, and/or leaves that conspicuously clasp stems.  But unlike Lactuca species, Sonchus species sometimes have involucral bracts and peduncles with minutely stalked glandsSonchus cypselae are also flattened, but not as strongly as those of Lactuca species.  Sonchus cypselae do not have beaks, though their cypselae may be narrowed apically.  Because cypselae of Lactuca canadensis have extremely short beaks, it is better to distinguish between the two genera based on head size.  The flower heads of Sonchus species are much larger than those of Lactuca species.  Sonchus species have heads with at least 80 florets, whereas heads of  Lactuca species contain only from 10-50 florets.  (Just count the ligules or styles.)

 

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Page updated 2016.07.18