8] MODIFICATION AND DISTRIBUTIONS OF CROPS 219 



A good example of weeds closely associated with particular crops 

 is afforded by the so-called ' linicolous ' plants that accompany Flax 

 (Limim). These appear to lack the normal adaptations for accom- 

 modating their development to seasonal changes, and may even be 

 dependent for the completion of their life-cycle upon being gathered 

 with the Flax crop when their seeds are ripe, kept in a storehouse 

 through the winter, and sown on open soil the following spring. 

 In extreme instances the plant has become so modified through 

 long association with the crop that its wild progenitors are unknown, 

 as is the case also with some crop plants. Examples of such weeds 

 of uncertain ancestry infesting cultivated Flax (as indeed their 

 specific epithets indicate) are a Campion, Silene linicola, and a 

 Dodder, Cuscuta epiliniini. There is thus not merely a very close 

 association but also a tendency to parallel variation between many 

 crops and some of their more commonly accompanying weeds, e.g. 

 through their disseminules being difficult, or mechanically impossible, 

 to separate from those of the crop itself. 



Naturalization and Acclimatization 



Although, in general, weeds tend to be hardy and to have a very 

 wide range of tolerance to differing environmental conditions, so 

 that they can spread far and rapidly, at least in ' disturbed ' areas, 

 crops are often fastidious in their habitat requirements. Both have 

 accompanied Man in his migrations over the world, however, and 

 from time to time have given rise to ' escapes ' or, more rarely, have 

 become established as naturalized aliens. But it is one thing to 

 escape from cultivation or a cultivated area into adjoining terrain, 

 perhaps repeatedly and under the beneficial influence of Man, and 

 quite a different problem to become sufficiently acclimatized to hold 

 sway in a fully wild state in undisturbed habitats among the local 

 natives. This latter is a relatively rare feat, as we shall see. Indeed 

 for the most part not only crops but also weeds are limited to areas 

 that are, or recently have been, in some way disturbed by Man. 



It is sometimes useful when dealing with plants transported out 

 of their normal areas to distinguish between naturalization, in which 

 they grow under natural conditions that are similar to those to which 

 they have been accustomed, and acclimatization, in which they are 

 adapted to new environmental conditions differing markedly from 

 those of their native habitat or habitats. Although instances of at 

 least some degree of the former are common and widespread, there 



