MORPHOLOGY OF VEGETATIVE STRUCTURES
VEGETATIVE ANATOMY (contributed by Richard Keating)General comments: Philodendron has secretory ducts present in all morphological parts. These occur mostly as 2-4 "files" in vascular bundles. Laticifers are of the non-anastomosing type, simple and articulated (French, 1988). The genus also has secretory ducts present in roots, stems, leaves and inflorescences. Roots have a sclerotic hypodermis.
Leaf surfaces: Cuticle smooth or occasionally rough on both surfaces. Epidermis: adaxial cells polygonal, straight-sided (1-2:1 l/w). Abaxial cells similar to or larger than adaxial cells or more elongate (2-4:1 l/w). Stomata: abaxial, randomly oriented, brachyparacytic to brachyparahexacytic. Venation: Secondary veins parallel with reticulate higher order veins.
Leaf Cross Section: Cuticle of medium thickness, may intrude deeply along anticlinal walls of epidermal cells. Abaxial cuticle may be thinner and smooth or striate. Epidermal cells: large or small, square to columnar or tile-like adaxial cells, larger abaxially, or same size; outer and inner walls flat. Stomata level with surface with conspicuous double cuticular flanges often present. Hypodermis: absent or as 1-3 layers present on adaxial side of midrib, or as isodiametric cells beneath both epidermal layers (in P. crassispathum, hypoderm of 1-4 layers adaxially and 2-3 layers abaxially), cells may contain dark material. Mesophyll: palisade 1-4 layers of elongated or rounded cells, usually packed with plastids; occupying 10-30% of lamina. Spongy layers up to 15 cells deep. Air spaces: large, substomatal cylinders perpendicular to surface; from 30-90% space in spongy layer; or air cavities irregular in some species. Paradermally, spongy tissue with one layer of unarmed cells over abaxial epidermis, then small cells or short-armed cells surrounding large air cavities extending from stomata to palisade cells; 3-5 spongy cells per uniseriate partition. Large cavities in midrib separated by uniseriate partitions of ground tissue, partitions 3-4 cells long between junctions. Collenchyma: 4-5 discrete subepidermal layers of small cells on abaxial side of midrib, often extending to vascular bundles.
Vascular bundles: one to several large bundles in the midrib, often at corners of a net-like aerenchyma; well delimited, with 1-5 large metaxylem cells, next to a small, round or elliptic phloem strand. Sclerenchyma: fibers, usually as a phloem cap 1-3 layers deep, and less commonly as a xylem cap. Some laminar bundles ensheathed with fibers. Fibers having up to 5 discrete wall layers. Xylem cells often ringed with a single layer of small parenchyma cells. Secretory ducts: occasional in mid-lamina or midrib with 1-2 layers of epithelium. Laticifers: lateral to vascular bundles, 2-3 per bundle, outside fibers, often with short processes protruding between adjacent mesophyll cells; containing dark amorphous contents. Starch: abundant in midrib ground tissue, not seen in lamina. Crystals: raphides common or uncommon in single packets in short or elongate cells, ends protruding across partitions into both adjacent air spaces, or in large rounded cells of upper or lower mesophyll. Raphide packets occasionally surrounded by dark sheaths within the cell (in P. crassispathum crystal cells elongated parallel with vascular bundles). Druses rare or common, often coarse and sharp-pointed, in rounded cells in palisade and spongy tissue partitions. Crystal sand absent or common in most mesophyll cells.
Petiole Cross-Section. Cuticle smooth and thin. Epidermis of small cells, level or rounded on surface and inner walls rounded to angular. Hypodermis of 2-4 layers. Collenchyma beneath hypodermis as a continuous band of 4-8 layers of angularly thickened, longitudinally elongated cells (8-10:1 l/w). Chlorenchyma: up to 5 rows of cells inside collenchyma, and in addition, in P. jodavisianum, as perpendicular partitions dividing the collenchyma band, and reaching subepidermal layers. Ground tissue of various sizes of parenchyma cells among numerous small to medium-sized air cavities separated by uniseriate partitions of up to 3 cells long. Cells often longitudinally elongated (3-6:1 l/w). Vascular bundles numerous and scattered, mostly oriented with phloem facing perimeter. Bundles are parallel in longitudinal section with some higher order angular connections. Xylem of 1-3 protoxylem cells per bundle, with spiral to annular thickenings, and occasionally one metaxylem element. Phloem of sieve cells and companion cells forming organized grid. In longitudinal section, sieve elements with horizontal to 45ðE oblique end walls. Sclerenchyma: fibers forming phloem cap, 2-4 cells deep, usually not fully surrounding phloem laterally. In some species, fiber caps surrounding xylem and phloem or ensheathing entire bundle.
Secretory ducts surrounded by epithelium and 1-2 rows of additional small cells, very common in collenchyma and common in ground tissue. Laticifers: 1-3, found laterally on one or both flanks of vascular bundles; non-articulated. Starch common throughout ground tissue, rare in collenchyma. Crystals: narrow raphide cells occasional, oriented across aerenchyma partitions with ends pointing into air cavities, or cells randomly oriented. Druses present subepidermally and in cells bordering air cavities. Small groups of small prismatics common in ground tissue.
Stem Cross-Section: 1-3 cm diam. Cuticle of medium thickness, smooth to rough-surfaced. Epidermis of very small cells, outer walls rounded. Cork: cells may be present subepidermally. Hypodermis: a subepidermal layer of large cells, over 1-2 layers of thick-walled fibers, followed by thin-walled periderm cells. Collenchyma: angularly-thickened cells, 3-10 layers deep, beneath periderm. Cortical ground tissue of loosely packed parenchyma. Air cavities: cell-sized or smaller in all ground tissue. Central cylinder: weakly delimited by a circle of vascular bundles, some of which tangentially fused in pairs or threes by phloem fibers and confluent phloem strands. Cortical bundles randomly scattered. Vascular bundles collateral and highly variable in form and orientation. Compound bundles numerous in central cylinder with irregular fiber strands in center of groups of 2-6 bundles. Small phloem strands outside fibers facing groups of 1-2 metaxylem elements on outer side. Simple collateral bundles may have a large wide strand of fibers surrounding phloem, a very small irregularly shaped strand of fibers, or no fibers. Fibers having up to 7 discrete wall layers visible.
Secretory ducts small and numerous in collenchyma and outer cortex, less common elsewhere. Duct cavities surrounded by two modified layers of small cells; contents of cavities testing positively for lipids. Laticifers: absent or present as 1-3 cells lateral to many vascular bundles, occasional in phloem. Crystals: raphides as single packets in elongate cells, most common in central cylinder. Druses of all sizes common in small cells in cortex.