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ADA HAYDEN 
chyma 3-4 cells deep bound the perimeter of the vascular cylinder and surround 
the hollow center. 
Summary: Parenchyma poorly developed; air space small; mechanical tissue prom- 
inent; vascular bundles large, strongly sheathed. 
Cyperaceae 
Scirpus fluviatilis (Torr.) Gray. 
Habitat : Alluvial basin ; swamps. 
Gross structure : Rhizome elongated, terminating in tuber-like swellings ; radial growth ; 
moderately thick; hard tuberous tip but spongy root -stalk; horizontal, descending; 
perennial (Plate XV, fig. c). 
Histology of rhizome (Plate XIX, fig. 3): 
Primary cortex: Radius 1/2 that of the rhizome; thin-walled, angled to oval 
parenchyma cells form a band 1/5 the depth of the cortex and send down rays 
to the vascular cylinder, forming arches around the large air spaces; endodermis 
cells distinct, of uniform thickness. 
Stele: Just inside the endodermis one row of concentric vascular bundles strongly 
sheathed with wood fibers; woody sheathed bundles scattered through the par- 
enchyma; parenchyma compact; thin-walled. 
Summary: Air cavities prominent in cortex; parenchyma thin-walled; vascular bundles 
numerous; parenchyma fairly prominent in central cylinder. 
Scirpus validus Vahl. 
Habitat: Alluvial basin; margin of pond. 
Gross structure: Rhizome stout, scaly; horizontal; linear extension; cortex soft and 
absorbent; vascular cylinder hard and flinty; fringes of slender roots with secondary 
hair-like branches radiate from the short nodes, appearing continuous not clustered; 
perennial (Plate XV, fig. g). 
Histology of rhizome : 
Primary cortex: Radius 1/3 that of the rhizome; air space prominent; parenchyma 
cells round, with 5-6 spoke-like, short cellular flanges forming a characteristic 
network; endodermis prominent with thick inner walls. 
Stele: Solid; pith parenchyma cells round to oval, rather thick-walled; concentric 
bundles numerous around the outer perimeter of cylinder, but larger and fewer 
toward center; bundles thickly sheathed by centripetal arc of bast. 
Summary: Aerenchyma prominent; this type of cell structure not only provides ample 
air space but insures mechanical strength; vascular tissue prominent. 
Iridaceae 
Iris versicolor L. 
Habitat: Alluvial basin; margin of pond, swamp. 
Gross structure: Rhizome thick, relatively short; tuberous swellings; horizontal; linear 
extension; roots slender, vertically descending, clustered at nodes; fine secondary 
branches; perennial (Plate XV, fig. e). 
Histology of rhizome (Plate XIX, fig. i): 
Primary cortex: Radius 1/4 that of rhizome; 3 rows hypodermal mechanical tissue; 
a similar band of angular cells with slightly thickened walls bordering the inner 
arc of cortex; aerenchyma a hexangular network; endodermis thick- walled on 
inner surface. 
Stele: Pith parenchyma thin-walled, compact, forming a small cylinder in center 
not entered by bundles; vascular bundles concentric, numerous around perim- 
eter of cylinder, farther apart but larger toward center; woody sheath prominent. 
Summary: Cortex with prominent aerenchyma; vascular bundles strongly reinforced. 
Histology of root (Plate XIX, fig. 2): 
Primary cortex: Radius 5/6 radius of root; hypodermis, 2-3 rows of collenchyma; 
