E. V. Wulff — 1 10 — Historical Plant Geography 



KUPFFER (1925) points out that the Baltic ports in former times 

 were regions of the concentration of adventive plants brought by ships 

 together with their ballast. In more recent times, the loading of 

 ballast on ships having been entirely discontinued, all this flora has 

 disappeared, with the exception of a very few species, such as Diplo- 

 taxis muralis and D. tenuifolia. Its place was then taken by a different 

 adventive vegetation distributed along the railways, at grain ele- 

 vators, etc. During and after the first World War the through transit 

 of freight from Russia into the Baltic countries almost entirely ceased, 

 causing the disappearance of many species from this vegetation. Dur- 

 ing the war years, 1915-1917, there made their appearance very many 

 new adventive plants brought together with provender from southern 

 Russia. After a few years these newcomers likewise almost completely 

 disappeared, being crowded out by species of the local flora, that again 

 took possession of their former habitats as soon as they were no longer 

 so trodden down by great masses of men and horses. 



These phenomena of the sudden disappearance of adventive species 

 that had already apparently acquired full rights of citizenship, if not 

 within the local flora, in any case within the limits of the territory culti- 

 vated by man or surrounding his dwelling places, may be ascribed to 

 the incomplete naturalization of these plants. The occurrence of an 

 extreme variant of any one of the local habitat conditions sufi&ces to 

 destroy these plants altogether or to injure them to such an extent 

 that they lose out to their competitors in the struggle for existence. 



How diverse are the habitat requirements of a plant and how difh- 

 cult it is for us at once to understand the reason for its incomplete 

 naturalization may be seen from the following two examples of sterility 

 in cultivated plants reported by Cammerloher (1927). TImnbergia 

 grandiflora (of the family Acanthaceae) grows wild in India, where it 

 propagates itself freely. In Java, on the other hand, where it was 

 introduced because of its large, beautiful flowers, it never fruited, 

 despite the presence of the necessary pollinators. Artificial pollination 

 also proved of no avail This seemingly inexplicable phenomenon 

 found its explanation in the fact that all the specimens of this plant 

 growing in Java had arisen by vegetative propagation from a single 

 plant brought from India and, consequently, constituted one single 

 "clone". The absence of fruits was, therefore, due to the self-sterihty 

 of this clone, which was proved by the introduction of a new specimen 

 from India, whose pollen induced the development of the ovaries of the 

 flowers of the old, hitherto fruitless specimens. As a second example 

 let us take Aristolochia arborea, three specimens of which were brought 

 from Mexico to the Buitenzorg Botanical Gardens (Java). In their 

 new home these trees flower abundantly during almost the entire year, 

 but they fail to bear any fruit. There is no lack of pollinators, and 

 artificial pollination is fully successful. The cause of sterihty in this 

 case has proved to be the fact that for the biological development of 

 the given plant the climate should not be too humid. The pistils and 

 stamens complete their development at different times, the former a 

 day before the latter. Observations in Java of the flowers at the 

 time of opening showed that, when first open, the flowers are already 

 in the second (staminate) stage of their development, while at the 



