Bothriochloa ischaemum

Bothriochloa ischaemum var. songarica — King Ranch Bluestem
Morphological Characteristics

by Bob Harms email-here

Habit

A dense population of younger B. ischaemum plants.

The typical description of B. ischaemum as a 'bunch grass' is somewhat misleading, especially in comparison with local native bunch grasses such as Schizachyrium scoparium (little blue stem), which forms clearly defined tufts at all stages. As shown above, the densely distributed plants, crowding possible competitors, aren't exactly bunches. But in time the stonger plants will crowd out adjacent relatives and form clearly defined bunches, often with open space in between, as below:

Old growth B. ischaemum with Ruellia nudiflora (wild petunia – green forbs, July 2007).

When repeatedly mown, it sprawls horizontally, sending up culms only in bloom. (I have not found stoloniferous or rhizomatous forms in our area.)

Sprawling.	Next to S. scoparium (tall, bright green), both recently mown (July 2007)

Leaf and Culm Features

Perhaps the most distinctive feature of B. ischaemum is the papilla-based hairs on the blade just above the collar (where the sheeth and blade meet). This generally permits identification of a young grass prior to bloom. The culm node is ringed with short hairs, thus distinguishing it as variety songarica, King Ranch bluestem. (With some native Bothriochloa, the node hairs are prominent and distinctive.)

Culm, sheath & blade	Ligule	Node (var. songarica)
Papilla-based hairs.		B. barbinodis = 'beard node'

Inflorescence

Although B. ischaemum's inflorescence often appears to be digitate, it is a panicle of racemes (branches) which bear the spikelets. Distinctive of B. ischaemum is that the branches are longer than the panicle axis.

The racemes are often compressed along the panicle axis, opening only with maturity. Opening is controlled by small reddish swollen organ, the pulvinus (plural pulvini), at the base of each branch. At anthesis the spiklets are also closely appressed, and the awns have yet to develop their bent (geniculate), twisted form (contrast the two images on the right).

Pulvini	Inflorescence detail

Spiklets

Spikelet morphology of Bothriochloa, as with other grasses that are commonly called 'bluestem' such as the native 'little bluestem' (Schizachyrium scoparium), is quite complex. (These grasses are in the tribe ANDROPOGONEAE. Named for the hairy spikelets, Greek andros 'man' and pogon 'beard.' An excellent presentation the spikelet morphology of this group is presented in The Flora of North America, vol. 25, available online as Manual of Grasses for North America; ANDROPOGONEAE). [Analysis of a similar spikelet for S. scoparium.]

The most salient and clearly visible feature for B. ischaemum is the arrangement of spikelet pairs along an axis with an abscission layer between each pair. Each pair consists of a sessile spikelet, with an awn, and a rudimentary sterile spikelet, usually smaller, on a pedicel. When mature the raceme bearing the spikelets disarticulates at the abscission layer, in such a way that a spikelet pair plus the associated axis section separate as a unit.

Disarticulation schema A = axis segment, F = sessile (fertile) spikelet, P = pedicellate spikelet	Spikelet pairs (adaxial view with axis section on the left)

The visible portion of the fertile spikelet (as shown above) are its glumes, the lower glume having numerous veins, and the awn, which extends the midvein of the fertile lemma (hidden within the glumes). The lower glume tightly clasps the upper glume, making it extremely difficult to dissect the spikelet to view the internal structure of the two florets inside.

Single axis section

Seed (Spiklet) Dispersal and Germination

The hairs of the spikelets provide virtually no support for dispersal by wind, nor are the plants generally tall enough for wind to carry the spikelets very far. Note that the pedicellate spikelet has no pubescence (unlike S. scoparium). Dispersal of spikelets seems most effective when they fall on relatively flat open surfaces, such as after a burn or onto a road surface, where wind can move them about, until they encounter established grassy areas.

Germination would seem to require exposure to light, again provided by burns, mowing, and grazing. The initial growth at germination may also require this exposure.