Trimorphic & Bimorphic Heads
by Bob Harms  email-here


C. texana trimorphic head
C. carduacea trimorphic head

C. texana with 3 floret series outlined
C. texana showing counts for ligulate and central florets

The early spring heads of C. texana & C. carduacea consist of 3 series of flowers (thus trimorphic):
  1. a peripheral series of ligulate (ray-like) pistillate flowers (which may be greatly reduced or even missing — heads lacking this series are said to be bimorphic) [above A]
  2. a central group of perfect flowers (similar to disc flowers), five–lobed, with exserted anther tubes [above C]
  3. an intermediate group of eligulate pistillate flowers with tubular corollas (generally not more than than half the length of the style), typically 50–60 percent of all florets. [above B]

A Fourth Peripheral Type with Closed–heads: Spring Florets with Reduced Ligules — Still Trimorphic

With one population of C. carduacea early spring ligulate florets were greatly reduced, hidden within the involucre and not apparent without dissection. This type was consistent throughout the population. (A mid–April head with even more greatly reduced ligules.)
C. carduacea closed–head type with reduced ligules
A: floret encased in pappus, but crimson color of old ligules peeks through; B: young floret; C–D: older crimson ligules.
In late spring and during fall/winter phase plants of both species exhibit peripheral florets with reduced ligules. In one instance a C. texana plant in cultivation with plenty of moisture initially bloomed with large ligules and then in mid April began to produce closed heads with similarly reduced ligules, as did the many subsequent blooms well into summer, and did not produce the early spring long, spreading ligules. The trigger for this shift is clearly not soil moisture, as I might have suspected this spring, when the area is experiencing record severe drought. But it could be related to record high temperatures, which began in April causing high transpiration rates. On the other hand, fall and winter C. texana plants behave the same way.

A Fifth Peripheral Flower Type: Long–tubular Florets — Trimorphic or Bimorphic?


In late spring and during fall/winter phase plants of both species exhibit peripheral florets with no ligules, but instead with long–tubular pistillate corollas, the filiform tube having two lips and with a length greater than half the length of the style.
For details, discussion & images.

Bimorphic Heads

Very few bimorphic heads have been found. The existence of long–tubular florets means that the absence of apparent ligules does not imply bimorphism; contrary to currently literature; e.g., Nesom 1995 (key on p. 157, leading to C. texana & C. nutans):
Flowers trimorphic or ligulate pistillate flowers lacking and then flowers dimorphic;
It is generally necessary to dissect the head, carefully separating the peripheral florets. Especially with pressed herbarium specimens having heads lacking apparent ligules it is extremely difficult (and at times impossible) to determine whether a head has reduced (vestigial) ligules, long–tublular florets or is truly bimorphic.

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