In a preliminary work: „A critical Revision of the genus Aristida”, I have given a review of all the hitherto described species of this genus with the citation of the literature, the exact copies of the authentic descriptions and the figures of the spikelet-characters, taken from the type specimens so far as I could locate them. In many cases it was necessary to enter into critical observations, because the nomenclature and the ideas found in the different manuals are exceedingly entangled. The Revision, although very important for botanists who wish to know the exact data of a fixed species, is not to use if we wish to determine an arbitrary plant of our genus, therefore we must have a monograph and I indicated already that it was my intention to write such a work. I must however observe that this work differs somewhat from other monographs and that it is in the first place a practical manual to the knowledge of this very difficult genus. It contains descriptions of all the species I have accepted as valid and keys for their determination. I was therefore obliged to omit in this work all the data already given in the Revision and to take into consideration that, with this monograph before us, we must, after being somewhat familiar with the genus and the different characters, without great difficulties, get a clear idea of it and with the keys before us find the name of a specimen belonging to our genus. It is therefore advisable to read the chapter where I have treated the different characters used for the limitation of the species. Anatomical characters are for practical reasons not taken up in the keys. No attempt is made to bring the allied species together in groups, because such groups are not easy to limit and the habit of such groups is scarcely to explain in a key. The keys to the species of each section are thus entirely artificial. On the other hand there are in our genus many very striking morphological characters of great constancy we can use with profit in the keys to recognize the species. American authors have divided the sections of a genus they studied, into minor groups, each group received a name, which was the plural of the most characteristic species of the group. In our genus we can give the names of „ripariae, cognatae, adscensiones"" purpurascentes, to the groups containing all the allies of Aristida cognata, purpurascens riparia, etc. The monographer recognizes these different groups often by indefinite characters of growth, colour or habit in general. I have therefore not accepted in this work the method of American botanists. Aristida is indeed a very difficult genus, not because the characters of the plants are difficult to understand, but because all the characters were taken hitherto — and there was no other way — from dried specimens, which are often damaged in course of time. I found an enormous diversity in the genus Aristida and although I studied about 15000 specimens, I could not expect to settle the characters definitively. No attention was given by taxonomists to the numerous intermediate forms, and hybrids were never observed or indicated in the literature of our genus. I am convinced that these hybrids occur in greater abundance than I have hitherto found in the different herbaria. For the knowledge of our genus in the future, agrostologists must study the species in the field and also by cultivation. Field study is very important when different species grow together and we can study and collect the intermediate forms. This was already done accidentally by some famous collectors, but no attention was given to the facts. Cultivation is also very important, not only to know somewhat more about the constancy of different characters, but also because we get quite undamaged specimens at our disposal. We know that the glumes and the awns are very fragile in our genus and that it is not always possible to give the different exact data of a species from herbarium-specimens. The different characters of the glumes and awns are in the future to verify with the living specimens.