Hypolytrum longifolium and its close allies make a large group, which is widespread and the most variable of all the taxa of Hypolytrum in South America. The H. longifolium complex as treated here involves all the plants hitherto classified under such names as H. irriguum, H. longifolium. H. nicaraguense, H. rubescens. and H. sylvaticum. Neither the variations in these five species nor the differentiation among them has ever been studied thoroughly.
Detailed comparisons of the variations in both floral and vegetative parts have been made on 197 specimens of the H. longifolium complex kept in six herbaria (F, GH, K, M, NY, US). The fructifications, which are considered to provide the most important taxonomic character in Hypolytrum, are very similar in shape and size in all of the five species, suggesting that the H. longifolium complex may possibly represent a single species. H. nicaraguense is sometimes separated from
H. sylvaticum and its allies by the fructifications that are clearly puncticulate with minute resinous spots, since such puncticulations are supposed to be absent in the other plants of the complex. However, several specimens from British Guiana and Venezuela, which can be identified as H. sylvaticum in their general appearance, bear clearly dotted fructifications, demonstrating that the presence or absence of resinous spots does not always warrant taxonomic separation among the taxa of the H. longifolium complex. The fructifications in H. nicaraguense also tend to have a little more elongated beak than in the other four species, yet they do not deviate from the range of the length of the fructifications measured in H. sylvaticum.
Certain vegetative characters appear to have a degree of significance in differentiating the taxa of the H. longifolium complex. Different colors of the basal leaf sheaths are pale brown to straw-colored in H. irriguum, H. longifolium, and H. sylvaticum, light purple in H. rubescens, and orange-cinnamon in H. nicaraguense. A few specimens of H. irriguum have light purple basal sheaths, which connect H. irriguum and H. rubescens as to the color of basal sheaths. In spite of this intermediate color of the basal sheath, H. rubescens differs from H. irriguum and all the rest of the complex by its leaf blades that are purplish beneath. All members of the H. longifolium complex except H. rubescens have the leaf blades pale green beneath. The orange-brown color of basal sheaths in H. nicaraguense is most distinctive.
The three species, H. irriguum, H. longifolium, and H. sylvaticum, cannot be differentiated by the color of the basal leaf sheaths. They are, however, separable to some extent, by the width of leaf blades. As shown in Table IV, the leaf width of 163 specimens from a mixture of the three species showed a trimodal distribution having peaks at 3.0-3.9 mm, 7.0-7.9 mm, and 11.0-11.9 mm. The first, the second and the third peaks represent H. irriguum, H. longifolium, and H. sylvaticum respectively. The peak at 11.0-11.9 mm stands clear from the other two, demonstrating that H. sylvaticum may be defined as having leaves more than 10 mm wide. The first and the second peaks, however, are so close together that they may be merged into one peak when more specimens become available for measurement. In addition to this statistical evidence, the localities of H. irriguum show dry, rocky conditions suggesting that H. irriguum may possibly be an ecological variant of H. longifolium. It is extremely difficult to segregate these two species using herbarium specimens.
In conclusion, none of the five taxa of the H. longifolium complex is clearcut, but four of them, H. longifolium, H. sylvaticum, H. rubescens, and H. nicara-guense, may be differentiated to some extent by leaf characters. I propose to treat these four plants as subspecies of H. longifolium.