Chapter VII —107— Artificial Factors 



and then not completely, while the very closely related species, B. 

 racemosiis and B. pratensis, have such brittle spikes that they break up 

 into separate spikelets very readily. Moreover, the awns in B. secalinus 

 are much reduced in length or rudimentary, which is undoubtedly con- 

 nected with the loss of abiHty and need for independent distribution. 

 Aleclrolophus apterus, a weed infesting rye, has lost the ability of self- 

 dissemination due to the fact that its seeds do not have any wing-like 

 appendages. Its seeds ripen simultaneously with the rye seeds, and 

 they are, thus, harvested and sown together with the latter. 



Loss of those adaptations making independent existence possible 

 leads in some cases to the plant ceasing altogether to grow in a wild 

 state and to its becoming so closely linked with some crop plant that 

 it is as difficult to determine the wild progenitors of such a cultivated 

 weed as it is in the case of many crop plants. As examples of such 

 weeds of unknown ancestry we may cite two infesting cultivated flax — 

 Silene linicola and Cuscuta epilinum. 



The above facts serve to illustrate one of the limitations to the 

 significance, from a botanico-geographical standpoint, of man's part in 

 the distribution of plants. But this is not the only limitation. 



Acclimatization. — Of no less importance, as regards the possibility 

 of plants becoming established in new localities into which they are 

 artificially brought by one or another agency, are the climatic and 

 edaphic limits beyond which a given plant cannot exist. Alphonse de 

 Candolle made a distinction, in cases where plants are transferred to 

 new regions, between the acclimatization of plants, when they find them- 

 selves where conditions differ markedly from those of their native habi- 

 tat, and the naturalization of plants, when they are brought into a 

 region with habitat conditions similar to those to which they have been 

 accustomed. In case of complete acclimatization or naturalization, a 

 plant, in his opinion, gradually acquires all the properties inherent to 

 wild species, such as the ability to grow and to multiply by natural 

 means of propagation, both sexual and asexual, without man's aid. If 

 such a plant maintains itself for several years, during which there occur 

 climatic phenomena to which it has not been accustomed, we may 

 regard it as fully acclimatized. In other words, such a plant would 

 in no way differ from an indigenous one, and its artificial introduction 

 into an alien flora may be estabhshed only on the basis of historical 

 data. 



It should be noted, however, that, with the exception of weeds, 

 very few instances are known of the full acclimatization of a plant 

 occurring during the history of man. De Candolle, as a negative 

 example cites the date palm, which has been known to man for thou- 

 sands of years and which, despite many attempts to extend its area, 

 cannot be successfully grown beyond the limits of its cHmatic mini- 

 mum. Although it may be grown at more northerly latitudes than its 

 native home, e.g., in southern Europe on the shores of the Mediterra- 

 nean, it either yields no fruit at all or fruits that fail to ripen. 



Hence, with a few exceptions, we can speak about the acclimatiza- 

 tion of plants only in the sense of the gradual adaptation of a plant 

 to changed habitat conditions over a very prolonged period of time, 

 no doubt exceeding that span during which man has acted as a factor 



