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The primary lateral veins (referred to by some authors as "secondary veins") are those which branch off the midrib and extend to the margins usually without additional branching. This use of primary lateral vein is consistent with that of other earlier aroid workers, Engler, Krause and Sodiro. Engler and later Krause referred to the primary lateral veins as "nervis primariis." Sodiro also referred to the primary lateral veins as "nervis lateralibus I".

On larger cordate blades there is frequent branching of the primary lateral veins in the lower part of the blade where primary lateral veins are widely spaced (Fig. 26) [see also Croat & Bunting, 1979)]. The presence of these veins, which are referred to here as "secondary veins", is not commonly described but in shape and aspect they are virtually the same as the primary lateral veins (Fig. 24).

Both surfaces of the blade typically have primary lateral veins with shape similar to the associated midrib. Usually they are somewhat less prominently raised than the midrib. The primary lateral veins on the upper surface, while usually sunken, may be essentially flat with the surface. Sometimes they are raised but at the same time contained within a broad or narrow valley so that the general appearance is that the veins are sunken. Primary lateral veins may sometimes be "quilted" (Figs. 23, 200, 361), i.e., with the veins deeply sunken and with the intervening leaf tissue of the blade which lies between the primary lateral veins being broadly raised, making the surface appear like that of a plush quilt.

Primary lateral veins of P. subg. Philodendron are rarely either absent or so inconspicuous as to appear to be absent, such as P. brewsteriense, though they are sometimes not at all conspicuous, as in P. microstictum (Fig. 289) or P. sulcicaule (Fig. 403). Most species have three to six pair of primary lateral veins. These generally arise at an acute angle with the midrib and, after extending along or near the midrib, spread at an angle of generally 40-80 degrees toward the margins, generally forming a broad arc in the process. The angle of primary lateral veins for all species falls within a range which may be as little as 5°E to as much as 35°E. The largest of these categories is the 50°E-60°E angle group with eight species and the 60°E-70°E angle group (also with eight species). Most range groups of primary lateral vein angles (see below) had less than five species. Only two range groups, the 40°E-50°E group and the 55°E-65°E group, have as many as five species. Only three such groups, 45°E-55°E, 45°E-60°E and 50°E-65°E, represent as many as four species. Six groups are represented by three species, eight groups by two species and the remainder of the 18 range groups are represented by a single species each.

For the most acute angle in a range of angles it is apparent that the primary lateral veins fall predominantly within a range of 40°E to 70°E. Only two species have their most acute primary lateral vein departing the midrib at between 20 and 29 degrees. In the 30°E-39°E angle group nine species occur, while the 40°E-49°E angle group has 28 species, the 50°E-59°E angle group has 30 species, the 60°E-69°E group has 22 species, the 70°E-79°E group has two species and the 80°E-90°E group has 1 species.

For the most obtuse angle in the range, the group for 30°E-39°E has only one species, the group for 40°E-49°E has 9 species, 50°E-59°E has 21 species, 60°E-69°E has 27 species, 70°E-79°E has 22 species, 80°E-89°E has five species and there are only two species whose primary lateral veins spread at angles greater than 90°E.

The number of primary lateral veins vary from two per side in P. chirripoense to 25 pair in P. madronoense. While no other species in Central America is restricted to having just two pair of primary lateral veins, a number of other species, including P. morii, P. jacquinii, P. hederaceum, and P. glanduliferum, frequently have just two pair and two other species, P. morii and P. wilburii var. wilburii may sometimes have just two pair of primary lateral veins. In the former group, the 2-3 pair combination of primary lateral veins (with 3 species) is only slightly more common than those with 2-4 pair and 2-5 pair respectively.

There are 20 species in Central America with three or more pairs of primary lateral veins. Only one species, P. hammelii, is known to have only three pair of primary lateral veins while six species have 3-4 pair, two species have 3-5 pair, six have 3-6 pair and two species, e.g., P. radiatum and P. schottianum, have 3-8 pair of primary lateral veins.

There are 21 species with four or more pair of primary lateral veins with one specie (P. breedlovei) known to have only four pair, 4-5 pair (two species), 4-6 pair (four species), 4-7 pair (three species), 4-8 pair (P. immixtum) and 4-9 pair (P. sagittifolium) and 4-10 pair (P. tysonii).

Five or more pair of primary lateral veins are present in 19 species with only five veins known for one species (P. dwyeri) and 5-6 pair (six species), 5-7 pair (five species), 5-8 pair (two species), 5-9 pair (three species), 5-11 pair (one species P. strictum), 5-12 pair (two species) and 5-18 pair (P. strictum).

Species with six or more pair of primary lateral veins per side include P. cretosum with 6-9 pair, P. pirrense with 6-10, P. hebetatum var. hebetatum with 6-11 pair, P. heleniae with 6-13 pair and P. tenue with 6-20 per side. Others have 7-8 pair (two species), 7-11 (P. auriculatum), 7-15 (P. fortunense), 7-16 (P. wendlandii), 8-9 (P. utleyanum), 8-10 (P. folsomii), 8-14 (three), 8-17 (P. grandipes), 10 pair (P. roseospathum var. angustilaminatum, 10-12 pair (P. malesevichiae) 10-16 (P. davidsonii var. bocatoranum), 11-16 pair (P. copense), 17-20 pair (P. dolichophyllum), 18-19 pair (P. cotobrusense), 18-21 pair (P. davidsonii, and about 25 pair (P. madronoense).