Morphology and anatomy of grasses in Tribe Andropogoneae

Introduction

The purpose of this page is to introduce you to the basic structure of grass plants in the Tribe Andropogoneae, the Bluestem or Sorghum Tribe, including their morphology (form) and anatomy (internal structure). Generally speaking, andropogonoid grasses (meaning grasses in tribe Andropogoneae) are organized like other grasses. Therefore, the emphasis of this page is on their distinguishing features, particularly the structures of their leaves and reproductive parts.

For background on the general structure of the grass plant, visit the following page:

Leaves

Leaf morphology

Morphologically, the leaves of andropogonoid grasses are similar to those of many other grasses. The leaves are divided into sheaths and blades. The leaf sheaths are open, meaning that the edges of the sheath overlap but are not fused to one another. The leaf ligules are membrane-like or made up of hairs. 

Leaf anatomy

Leaf epidermis

As in other grasses, the leaf epidermis of andropogonoid grasses is made up of rows of cells that parallel the length (the long axis) of the leaf. These cells include long cells, short cells, bulliform cells, and the guard cells and subsidiary cells surrounding the stomata (pores).

Some short cells are silica cells that contain silica bodies. Silica bodies or phytoliths in andropogonoid grasses tend to be of the panicoid type, ranging from cross-shaped to bilobate (dumbbell-shaped) to polylobate.

Leaf vascular bundles (veins)

Andropogonoid grasses are C₄ grasses. In C₄ grasses, the light reactions of photosynthesis take place in the mesophyll cells, whereas the carbon-fixing reactions take place in the bundle sheath cells. The mesophyll cells are arranged concentrically around the bundle sheath cells in a pattern known as Kranz anatomy (Kranz is German for wreath). Unlike some other C₄ grasses which have a two-layered bundle sheath, andropogonoid grasses have only a single layer of bundle sheath cells with chloroplasts.

Reproductive structures

Grasses in Tribe Andropogoneae may be dioecious (have male, or pollen-producing, and female, or ovule-producing, florets on different plants), monoecious (have male and female florets on the same plant), or may have bisexual florets (florets with both male and female structures).

Inflorescences, spikelets, and florets

The inflorescences of members of Tribe Andropogoneae may occur at the ends of stems or in the leaf axils (the upper angle between leaf and stem) on the sides of stems. The overall structure of the inflorescence varies, but, in general, inflorescences are made up of clusters of branches. These branches may diverge from a single point at the end of a stalk or may branch off the sides of a central inflorescence axis.

Spikelets typically occur in pairs on branches. They may also occur in groups of three (triplets), particularly at the ends of branches, or, sometimes, one at a time on a branch. The inflorescence branches that bear the groups of spikelets are called rames. Rames usually disarticulate (break apart, also called shattering) when the fruit is mature. Some botanists consider rames to be a unique to Tribe Andropogoneae.

Spikelet pairs may include two pedicellate (stalked) spikelets, one sessile (stalkless) spikelet and one pedicellate spikelet, or two sessile spikelets. Typically, two glumes occur at the base of each spikelet. Spikelets may be bisexual (both male and female, or producing both pollen and ovules), unisexual (female or male) or sterile. Sterile spikelets produce neither pollen nor ovules, and their reproductive structures do not develop fully, if they develop at all; typically, sterile spikelets consist only of bracts (for example, glumes, lemmas, and paleas).

Each spikelet bears two florets. As with spikelets, individual florets may be bisexual, female, male, or sterile. The lemmas of the florets may be thin, and the paleas thin or even absent. Bisexual and male florets have three anthers.

Dispersal units

In wild members of Tribe Andropogoneae, the rames (inflorescence branches that bear the spikelets) disarticulate (shatter or break apart) when the fruit is mature. If spikelets are solitary on a rame, the rame will break beneath each spikelet. If spikelets are paired or in groups of three (triplets), the rame typically breaks apart at the base of each pair or triplet of spikelets. Thus, each spikelet pair or triplet is dispersed as a unit, often with a small segment of the rame (branch) still attached. 

In domesticated members of Tribe Andropogoneae that are grown for their grain, like maize (Zea mays) and domesticated sorghum (Sorghum bicolor subsp. bicolor), the inflorescences often do not shatter (break apart). Non-shattering inflorescences were selected for during domestication, probably to make grain easier to harvest and to prevent the loss of grain.