The stems of grasses growing in wet or marshy situations differ in structure from those detailed above. As examples the stems of _Panicum flavidum_, _Panicum colonum_, _Panicum Crus-galli_ and _Panicum fluitans_ may be considered. The stem of _Panicum flavidum_ is broadly ovate in cross section with a flat front and is more or less solid, though occasionally the parenchymatous cells in the centre get broken. Two rows of vascular bundles surround a fairly large amount of parenchymatous cells of the ground tissue. There is a continuous ring of sclerenchyma separated from the epidermis by a fairly broad cortex. The cortex has a number of fairly large air-cavities separated by bands of parenchymatous cells. Within the sclerenchymatous band lie small vascular bundles at regular intervals just towards the cortex. A few isolated bundles are in contact with the inner border. (See figs. 37 and 38.) The stems of _Panicum colonum_, _Panicum stagninum_ and _Panicum Crus-galli_ have in their centre in the ground tissue stellate cells with air-cavities. This part is surrounded by a fairly broad portion of parenchymatous cells in which are imbedded two rows of bundles. Outside these bundles runs round the stem a narrow sclerenchymatous band with a few bundles in it of which some touch it inside and others outside. Two bundles are found by themselves in the tissue of stellate cells. In _Panicum Crus-galli_ three or four bundles are met with amidst the stellate cells. [Illustration: Fig. 40.--Transverse section of a portion of the stem of Panicum Crus-galli. x 70 1. Epidermis; 2. sclerenchyma; 3. vascular bundle.] [Illustration: Fig. 41.--Transverse section of the stem of Panicum stagninum. x 10] The cortex outside the band of sclerenchyma is full of air-cavities, small and large. In _Panicum colonum_ the outline of the stem is ellipsoidal with the front quite flat, and the cortex is narrow at the sides and very broad in front and at the back. The sclerenchymatous ring is circular in outline. The stem of _Panicum Crus-galli_ is broadly ovoid and the cortex is uniformly broad. The epidermal cells as well as the lower cells are thickened in the stems of _Panicum fluitans_ and _Panicum Crus-galli_, but in the stems of _Panicum colonum_ and _Panicum flavidum_ the epidermis alone is thickened. In the cortical portion outside the sclerenchymatous band, small vascular bundles occur in the stems of _Panicum colonum_, _Panicum Crus-galli_ and _Panicum fluitans_. (See figs. 39, 40, 42 and 43.) [Illustration: Fig. 42.--Transverse section of the stem of Panicum fluitans. x 15] [Illustration: Fig. 43.--Transverse section of a portion of the stem of Panicum fluitans. x 60 1. Epidermis; 2. sclerenchyma; 3. vascular bundle; 4. air-cavity; 5. diaphragm.]