The Science of Botanical Art

Grasses

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 17, Issue 3

 

I always hesitate to talk about the morphology of grasses. They are indeed flowering plants, and in the clade of the monocots, but their growth pattern and flowering style are so distinctive that there is a whole vocabulary designed to describe them. Not only that, but there are at least three families that have a similar grasslike appearance and that needs discussion, too. So hang on to your hats and do your best to follow through the maze of special terminology. I’ll try to make it as painless as possible. 

Part of the problem is the variety of forms that happen even within the definition of the inflorescence, so we will start with a ‘typical’ grass plant and load you with a very long list of qualifications. 

The grass plant consists of a jointed stem that is called a culm, and leaves, which are sessile and sheathed, and in turn consist of a blade and a sheath. At the junction of the blade and the sheath there is often an additional piece of tissue or hairy fringe on the inside (adaxial) that is called a ligule. In some genera the blade itself has extensions on either side of the ligule, and these are auricles (or ears). The back of this junction may have a ‘collar’. 

Most culms rise from a horizontal stem-like organ which in turn gives rise to the fibrous root system. When this grows below ground we call it a rhizome, if it grows above ground it is known as a stolon. In the perennial grasses, a mat of rhizomes gives rise to the sterile leaves that make up a lawn. One of the special characteristics of these leaves is the fact that the meristem – the actively dividing center of growth is not at the apex or tip of the blade as is the case in most plants, but at the base – an intercalary meristem, and this allows us to mow the tops off the grass without interfering with its continuing development. 

The stem of a grass is round in cross section (terete) and hollow or pithy, except at the nodes. This is one of the characters that distinguish grasses (Poaceae) from its cousins the sedges (Cyperaceae) that have solid stems that are triangular in cross section (sedges have edges) and the rushes (Juncaceae) that are terete but solid. 

Completely confused? And I haven’t even begun the reproductive organs yet. The floral unit of a grass is a spikelet. The spikelet of a grass has two scales at the base named glumes, again one folded into the other, but this time without different names – just first glume and second glume. Spikelets in turn can be arranged in spikes, racemes or panicles, and in grasses the latter is the most common. 

A spikelet itself is made up of a number (sometimes only one) of florets. The axis of a spikelet is a rachilla, one of those diminutive terms in botany that sets my teeth on edge – like spikelet and floret (rachilla is the term for little rachis). In most cases in Poaceae these are perfect but incomplete, lacking a perianth; no sepals or petals. Instead there are two bracts, one folded into the other, that hold the pistil and stamens. 

The outer bract is called a lemma; the inner and smaller bract is the palea. In some grasses there is a pointed extension of the midrib of the lemma called (you guessed it) an awn. I want credit for all the crossword puzzle answers I am handing you. 

Enclosed in the palea are the two reproductive organs. There are usually three stamens with the anthers basifixed, but pointed and divided at the base (sagittate), and there are mostly two hairy style arms arising from the pistil. At the base of the ovary there are two small leaf-like organs called lodicules that are responsible for opening up the floret when it is ripe. Some suppose the lodicules are the remnants of the perianth. 

The fruit of Poaceae is a caryopsis or grain – an achene in which the seed is fused to the pericarp. This happens to be one of, if not the, most important economic fruits of the plant kingdom, providing the basic foods of most of the worlds populations – rice, wheat, and corn for example. 

There are indeed three families that are grasslike in their habits. Besides the grasses there are the sedges and the rushes. Along with the differences in the stems, there are other features that you should be on the lookout for. The flowers of the sedges are more often imperfect and monoecious while still in spikelets, without the sterile glumes at the base. They too, lack sepals and petals, but have only a single bract. The inflorescences of sedges are umbels and heads and these can be interesting challenges to paint – but the easiest way to define the difference between the two groups is still the rolling of ones’ fingers around the stems to feel the ‘edges’. 

Rushes on the other hand are more grasslike in the stems but are apt to have solid leaves. They have perfect, complete flowers usually in umbels, with parts in threes, even though it might take a hand lens to discover them, and they look like miniature lilies. 

Here are again a group of plants without showy colorful flowers, but whose charms are subtler and more challenging, and defy making the most out of a limited palette, and the inherent delicacy of their habit. 

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