MANUAL OF THE GRASSES OF THE UNITED STATES 7 



VEGETATIVE ORGANS 



In size grasses vary from minute species only 2 or 3 cm high to 

 the giant bamboos 30 m tall. The vegetative organs, however, 

 consist, in all cases, of root, stem, and leaves. A single unbranched 

 stem with the attached leaves, is a shoot. 



ROOT 



The roots of grasses are fibrous with little modification. The pri- 

 mary root persists only a short time after germination, its place 

 being taken by secondary roots produced from the nodes of the young 

 culm. Besides the original root system at the base of the plant, 

 secondary roots are often formed from nodes above the ground as in 

 maize (prop roots), or from the nodes of creeping culms (rhizomes or 

 stolons). Roots are never produced from the internodes of the culms. 



STEM 



The jointed stem of a grass, called a culm, is made up of a series 

 of nodes and internodes. The internode is hollow (wheat), or solid 

 (maize); the node or joint is always solid. The culm may branch at 

 the base as in wheat (stools) or above the base as in Muhlenbergia. 

 Creeping culms, modified for propagation, may be below ground 

 (rhizomes) or above ground (stolons). The lower internodes may 

 thicken into corms (timothy, species of Melica, Arrhenatherum elatius 

 var. bulbosum), sometimes referred to as bulbs. Perennial grasses 

 may form a sod or mass of individuals by means of rhizomes or 

 stolons, or they may form a crown or tuft by the continual formation 

 of upright branches within the lower sheaths. 



LEAF 



The leaves are borne on the culm in two ranks, one at each node 

 The leaf consists of sheath and blade. The sheath envelops the culm 

 above the node, the margins overlapping (open) or infrequently 

 united into a cylinder for a part or a whole of the distance to the 

 summit (closed). 



The blades are typically flat, narrow, and sessile. In dry regions 

 they are usually involute or convolute ; in tropical shade they are often 

 comparatively short and wide (lanceolate, ovate, or elliptic) ; in most 

 of the bamboos they are narrowed into a short petiole articulate with 

 the sheath. 



Some grasses (especially the Hordeae) bear, one on either side at 

 the base of the blade, appendages known as auricles. At the junction 

 of the blade and sheath on the inside is a membranaceous or ciliate 

 appendage called the ligule. The region on the back of the leaf at 

 the junction of the sheath and blade is called the collar. 



PROPHYLLUM 



At the point where a branch shoot originates from a main shoot, 

 (in the axil of a sheath) there is produced on the side next to the 

 parent shoot a two-keeled organ (the first leaf of the shoot) called the 

 prophyllum. At first the prophyllum completely covers the bud but 

 later opens as the shoot develops. The organ is usually concave 

 toward the parent shoot but clasps the new shoot by its margins. 



