Validity of the Grass Species Digitaria adscendens

Abstract

The common crabgrass, Digitaria sanguinalis (L.) Scop., a native of Europe, has been introduced widely in many parts of the world and is often a troublesome weed. In the United States it is a serious pest of lawns, and according to Hitchcock (1950) is to be found throughout the country at low and medium altitudes. This species, as treated by most recent American botanists, is quite variable, and it has been noted that plants from the southern part of the United States and from tropical America differ somewhat from those from the northern part of the range. Nash (1912) considered the southern plant to represent a distinct species, Syntherisrna marginatum (Link) Nash ( _ Digitartia marginata Link) and they were also treated as specifically distinct by Small (1913) under the binomial Syntherisma fimbriatunm? (Link) Nash ( = Digitaria fimbriata Link). In recent years all American botanists have considered D. marginata and D. fimbriata to be only minor variants and have listed these as synonyms of D. sanguinalis. European botanists, on the other hand, have tended to recognize two species in this complex. Stapf (1919) accepted Digitaria marginata as a valid species; he provided an excellent description, and discussed the characters by which D. marginata may be differentiated from D. sanguinalis. Henrard (1934, 1950) also recognized both species, pointing out, however, that the correct name for the southern plant is P. adscendens (HBK.) Henrard, based upon Panicum adscendens HBK. which antedates D. marginata by six years. Because of the lack of agreement among agrostologists as to whether one or two species are involved in this complex, a careful study of the problem seemed indicated. Inasmuch as these plants are abundant in the Western Hemisphere, the study reported here is confined to plants from this region. The basis of the present studies was largely herbarium material; more than 300 collections from the Western Hemisphere were examined. In addition, plants were grown in the greenhouse from seeds procured from 25 localities representing 10 states as well as Canada and Mexico. These plants were studied to determine whether differences in growth habit could be detected. In general growth habit Digitaria sanguinalis and D. adscemdens are strikingly similar. Plants of the two species, grown in the greenhouse, were practically indistinguishable upon superficial examination. In both the culms are decumbent, rooting at the nodes, and the leaf sheaths, at least the lower ones, are papillose-pilose. The inflorescence consists of a number of racemes which are digitate at the summit of the culm; often 1-3 additional racemes are borne below. Those botanists who have recognized Digitarica adscendens as being specifically distinct have noted that in this species the spikelets are somewhat longer, the second glumes are usually two-thirds as long as the spikelets, and the nerves of