E. V. Wulff — 104 — Historical Plant Geography 



vegetation and habitat conditions — may be explained as a direct or 

 indirect effect of man's activities. And, as in the case of any such ex- 

 tremism, the most fantastic assumptions and unfounded statements 

 were regarded as scientific and well-founded. The bald statement that 

 the distribution of a given plant is the result of its introduction by 

 man was considered sufficient to refute all propositions regarding the 

 effect of historical causes on the present distribution of plants. 



What limits the significance of man's activities as a factor in the 

 geographical distribution of plants? First, the short duration, as com- 

 pared to the antiquity of the plant world, of the action of man on the 

 latter, and, second, the fact that, even conceding, as we do, the great 

 role of man in the transfer of seeds and the spread of weeds, this does 

 not determine the extent to which this vegetation is able, in places 

 where it penetrates into regions of the earth to which it is not native, 

 to gain a foothold in the local flora and become a full-fledged member. 

 The few instances of the almost cosmopolitan distribution by man of 

 some plants do not vouchsafe the unHmited possibility of the naturali- 

 zation of plants, regardless of what habitat conditions are normal for 

 them and what geographical obstacles may be in the way of their fur- 

 ther dispersal. 



Loss by cultivated plants (and by weeds infesting them) of their 

 ability to maintain an independent existence and to spread inde- 

 pendently: — First of aO, there arises the question as to whether a 

 plant, once in cultivation, can pass, after it is no longer cultivated, into 

 a wild state, i.e., can maintain an existence independent of man. In 

 other words, does not man's influence on a plant under cultivation 

 result in so changing its structure that it is, to a certain extent, de- 

 prived of the advantages in the struggle for existence that it had ac- 

 quired during the entire preceding period of its life history? As 

 regards those cultivated plants utilized by man for centuries and modi- 

 fied by breeding and selection to conform to man's needs, it is quite 

 apparent that the answer to this question must be affirmative. The 

 changes wrought by man in the structure of plants may be exceed- 

 ingly diverse. We shall here note only the most important, taking as 

 our chief sources the works of Zinger (1909), Hildebrand (1873), 

 and Thellung (191 5). 



1. Conversion of perennials into annuals, as an effect of cultivation, 

 is not an uncommon phenomenon. We know, for instance, among 

 cereals many cases of cultivated annuals that have originated from 

 wild, perennial forms. Thus, the wild progenitors of cultivated rye 

 were perennials (Batalin, 1892), and the wild rye, Secale montanum, 

 closely related to cultivated rye, is also a perennial. 



2. Increase in the size of seeds, accompanied by a decrease in their 

 number and, consequently, also in the number of embryos, greatly 

 lessens the plant's chances for propagation. 



3. Loss hy fruits and seeds of their protective coverings. For example, 

 naked cereals have fruits deprived of the outer glumes that served to 

 protect them. The lining of sclerenchymatous tissue in the pods of 

 wild Leguminosae is lacking in cultivated species. 



4. Loss of natural adaptations for the dissemination of seeds. Here 



