E. V. Wulff —106 — Historical Plant Geography 



or to other causes— results in these plants becoming incapable of com- 

 pleting their sexual cycles. 



When no longer cultivated, all plants such as we have enumerated 

 above — modified by man in ways most suited to his needs but ab- 

 normal and harmful for the plant itself as an independent organisni— 

 naturally tend to disappear and are no longer able to aid in extending 

 the area of the species to which they belong. 



Of exceptional importance is the circumstance that the very same 

 changes in biological peculiarities and morphological structure that we 

 have just described in plants purposely cultivated may also be ob- 

 served in those weeds that constantly accompany definite crops, thanks 

 to which they are involuntarily cultivated by man. Certain of such 

 weeds, without man himself being aware of the process, may gradually 

 become transformed into direct objects of his cultivation. This has 

 undoubtedly been the mode of origin of cultivated rye, oats, hemp, 

 and coriander, and most probably of several Leguminosae, kenaf {Hibis- 

 cus cannabinus), Chinese jute (Abutilon Avicennae) , etc. 



As proof of the existence of such involuntary breeding of weeds, 

 making possible their subsequent cultivation, we may cite some of 

 the numerous instances of the occurrence in weeds, the planned 

 selection of which by man is entirely excluded, of changes in structure 

 and biology analogous to those we listed above for cultivated plants. 

 Thelltjng (1915) compiled considerable data of this sort. Here we learn 

 of the involuntary selection of weeds for annual habit, due to the 

 annual plowing of the soil, and of the consequent transformation of 

 perennial plants into annuals, an example being Phalaris brachyslachys, 

 a Mediterranean weed differing only in being annual from its close 

 relative, P. truncata, that grows in natural habitats. In weeds, just as 

 in cultivated plants, we may observe an increase in the size of seeds at 

 the expense of their number, and the loss by fruits of their protective 

 coverings and of their ability to disseminate their seeds naturally. 



Examples of such changes may also be taken from Zinger's work 

 (1909) on species of Camelina infesting flax. C. glahrala and C. lini- 

 cola constantly accompany flax, being known as "plantae linicolae". 

 According to Zinger, these plants "do not have a single one of the 

 adaptations, possessed by most wild annuals, for accommodating their 

 development to seasonal changes. They are unable to await the 

 moment most favorable for the development of stem and flowers, hav- 

 ing neither rosettes like their nearest wild relatives nor seeds that may 

 lie long dormant like many other annual plants. Their fruits ripen 

 ahnost simultaneously, instead of successively over a prolonged period, 

 which would assure the preservation of at least that part of the prog- 

 eny attaining by the onset of winter a stage of development favorable 

 for wintering-over. These species can normally complete the cycle 

 of their development only provided their seeds are gathered when ripe, 

 kept in a storehouse during the winter, and sown in the spring." 



The well-known, widely distributed weed, Agrostemma gilhago, has 

 many-seeded pods, which, although dehiscent, are so constructed that 

 the seeds cannot fall out by themselves. According to Braun-Blan- 

 QUET, the spikes of Bromus secalinus, a weed infesting cereals, and its 

 close relative, B. grossus, break up into sections only late in the season 



