2l8 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



domestication of plants, as for example among the cereals. This 

 is advantageous from Man's point of view in speeding up crop- 

 production, and may favour the plants' own chances of survival in 

 ' open ' habitats such as those prepared for cultivation ; but it 

 places them at a great disadvantage in competition with natural 

 vegetation, which in most undisturbed land habitats is dominated 

 by perennials. 



(9) Absence of successful fruiting — for example due to atrophy of 

 the sexual organs, to absence of pollinators far from the plants' 

 native habitat, or to the desecrations of Man — results in such plants 

 being incapable of self-perpetuation by the usual means. 



(10) Seedless fruits — which are often an objective of plant breeders 

 for cultivation, likewise render a plant incapable of independent 

 existence unless it has some effective means of vegetative propagation, 

 in which case it will still lose the benefits of sexual reproduction 

 (such as hybrid vigour and the exchange of genie material). 



(11) Double flowers — involving for example the ' conversion ' of 

 stamens into petals — again render the plant incapable of self- 

 perpetuation by the normal means. 



(12) Loss of defensive adaptations — such as spines, thorns, hairiness, 

 and hardness — renders the plant defenceless against animal grazing 

 and, often, more susceptible to injury from excessive loss of moisture. 



The above features may occur already among wild plants as 

 abnormalities, but in cultivated strains they tend to become intensified 

 by Man's conscious or unconscious selection and, often, perpetuated 

 through his propagation and protection. When no longer cul- 

 tivated, or, in the case of weeds, enjoying the benefits of cultivation, 

 such horticultural or agricultural strains tend to disappear. Having 

 been modified by Man in ways most likely to suit his needs (but 

 at the same time harmful to the chances of persistence of the plant 

 as an independent organism), they are no longer able to help even 

 in maintaining the area of the species to which they belong, at least 

 in many cases in the absence of Man's influence. 



The weeds most notably modified through cultivation are those 

 that constantly accompany particular crops, thanks to which they 

 have long been involuntarily cultivated by Man. Some of them — 

 such as, apparently, the cultivated Rye — have become so transformed 

 as to be now themselves objects of cultivation. For in weeds, just 

 as in intentionally cultivated plants, there tend to be such changes 

 as increase in size of seeds at the expense of their number, and loss 

 by fruits of their protective coverings and abilities to disseminate. 



