1. The different explanations, found in botanical literature for meristic variation in flowers are reviewed and discussed. The result of these considerations is that only the original variation of Eichler is able to give a satisfactory explanation of the facts, i. e. the supposition that meristic variation in regular flowers is due to phyllotactical differences, whereas in zygomorphic flowers the same holds true, with the apparent exception that a pseudomeiomery may be caused by abortion or by fusion. 2. The rejected views are: the explanation of pleiomery by fission, dedoublement (either taken as a phylogenetic or as an ontogenetic process) and reversely meiomery by fusion; the explanation of meiomery by abortion of organs; the explanation of pleiomery by the intercalation of sectors in the floral receptacle, and reversely of meiomery by omission of such sectors; the explanation of pleiomery by coalescence of flowers, synanthy or gamogemmie. 3. On the basis of original variation the following facts, connected with meristic variation, may be explained: that generally high numbers vary more than low numbers; that nutrition has a marked influence on meristic variation; that in many species intermediate stages between one and two organs occur in the form of broadened or bilobed members; that these transition stages may be classed in a continuous series, from which however it is not to be ascertained whether the series represents the doubling of one, or the fusion of two phyllomes; that the middle stages of such a series are more varying in character than the first or last; . that supernumerary organs are more frequent in a calyx sector (episepalous pleiomery or epipetalous meiomery) than in a corolla sector (episepalous meiomery or epipetalous pleiomery); that terminal flowers in some plants have a different floral number from that of the lateral flowers of the same plant, a difference which is especially striking in terminal peloria, which are formed on a stem much exceeding the lateral branches in strength (Digitalis). 4. The Polygonaceous flower, which has played an important role in the literature on meristic variation is utterly unsuitable for this kind of investigation, so long as we cannot distinguish between the several processes affecting the floral number. 5. The flower of Lythrum confirms several of the facts enumerated above under 3. The observed twin flowers showed no transition to the pleiomerous flowers; this is to be expected when we discard the coalescence theory for pleiomery. The observed abnormal flowers made it clear that the irregularity of the vascular supply of the intermediate stages is greater than that of these stages themselves: this gives a new confirmation of the view that leaf traces are formed basipetally. The same abnormal flowers suggest the following conceptions of whorl formation in the flower. Every floral whorl is formed anew by the linking up of a certain number of preexistent phyllomes. The common isomery of flowers may be the outcome of the influence of lower whorls on the formation of higher ones. The morphogenetic forces are different in the different zones; whether a stamen is differentiated as episepalous or as epipetalous does not depend on the insertion of the stamen on a calyx or a corolla vein, but on the morphogenetic forces of the whorl to which it belongs. Once being determined, which happens very early. before the vascular tissue is induced, the episepalous stamen has a strong affinity for calyx veins, the epipetalous for corolla veins. If there are two more stamens than perianth leaves, the most common solution is that three of them are associated with a single calyx sector; if there are two stamens short, two adjoining veins usually remain free; in such a way in both cases the alternation of the stamen whorls is maintained as well as the insertion on the appropriate veins. In such cases where the number of stamens is odd, there is more chance than elsewhere, that one single stamen will show intermediate plastics, as a consequence of the fact that the formation of two whorls in the androecium meets with a difficulty at one point.