432 BOTANICAL GEOGRAPHY. 



floras. He states their object to consist in the investigation of the limits 

 of the distribution of the species within a larger district, and for this purpose 

 to compare them, as e. g. the flora of Wurtemberg with that of the adjacent 

 countries. In this manner he shows, that when a natural subdivision of 

 Germany is made, the flora of "Wurtemberg belongs to those of the adjacent 

 districts, and does not contain distinct centres of vegetation of its own. 

 Mohl distinguishes four separate regions : the fluviatile system of the Neckar 

 and the Tauber, the Black Forest, the rugged Alp, and the tertiary plain of 

 Upper Swabia. 



1. The Neckar region, lying between the Swabian Jura and the Black 

 Forest, in regard to the distribution of its plants, may be considered as a 

 portion of the Rhine district. The eastern limit to the occurrence of a 

 tolerable number of plants exists at the Jura ; but with the single exception 

 of Orobus albus (at Tubingen), no species is found from the Neckar to the 

 Tauber, which does not also exist in the valley of the Rhine. But the dis- 

 trict on this side is poor in comparison to the latter ; for " in correspondence 

 with a general phenomenon," as the bed of a river becomes narrowed, many 

 plants disappear which are common lower down the stream. The author 

 also gives a list of more than fifty species which prove this connexion, and 

 from which we shall select the following as characteristic forms of the Rhine 

 district : Helianthemum celandicum (vineale), Myagrum perfoliatum, Isatis 

 tinctoria, Diplotaxis tenuifolia and muralis, Altfuea hirsuta, Lathyrus hirsutus, 

 Rosa gallica, Helosciadium nodiflorum, (Enanthe peucedanifolia, Carum bul- 

 bocastanum, Crepis pulchra, Lactuca saligncf; Artemisia pontica, Centaurea 

 nigra, Heliotropium Europaeum, Calamintha officinalis, Mentha rotundifolia, 

 Parietaria diffusa, Spiranthes testivalis, and Scirpus mucronatus. Geologically 

 considered, the district of the Neckar and the Tauber belongs to the muschel- 

 kalk, lias, and the keuper formations. Of these, the muschel-kalk, as in 

 Thuringia, exerts the most important influence upon the distribution of the 

 plants, whilst the lias and keuper, being less homogeneous formations, favour 

 a greater chemical variation in the soil. A list of about 20 sand-plants con- 

 trasted with 100 plants belonging to the muschel-kalk, shows how the latter 

 augments the number of indigenous species to a greater extent than the other 

 formations. 



2. The Black Forest, the soil of which is derived from bunter sandstone or 

 Plutonic rocks, contains in the district of Wurtemberg but few Phanero- 

 gamia peculiar to it, whilst the higher elevations of these mountains, which 

 are also poor in plants, belong to Baden. Among the plants of the Black 

 Forest of Wurtemberg, excepting those which also occur in other regions of 

 the kingdom, there is not one which is not distributed over the greater part 

 of the mountains of Germany, so that e. g. all those mentioned, excluding 

 Crocus vernus, occur also on the Hartz. If we bring into relation with these 

 facts Kirschleger's general remarks upon the whole of the Black Forest (see 



