Chapter VII —105— Artificial Factors 



we may cite the many-jointed spikes of cereals (so-called "brittle" 

 spikes), a character common among wild species but absent or nearly 

 so in cultivated species, and the bipartite fruits of wild species of 

 Umbdlijerae, a character only very slightly expressed in cultivated 

 species. Plants which in a wild state have fruits that, when ripe, de- 

 lusce in one way or another, thus giving the wind a chance to disperse 

 the seeds, may lose this ability in a cultivated state. For instance, 

 the fruits of cultivated flax or of the opium poppy do not dehisce when 

 ripe, while those of the wild poppy. Pa paver Rhoeas, and of the wild 

 flax, Liniini crepitans, are dehiscent. 



The loss of the sclerenchymatous tissue in the pods of Leguminosae, 

 about which we have already spoken, lessens the extent to which the 

 valves of the pod curl up, thus diminishing the abihty of the plant to 

 scatter its seeds. 



In general, the lesser development of sclerenchymatous tissue in 

 many crop plants is a common phenomenon, apparently connected 

 with the fact that plants, when cultivated, are grown in closer stands, 

 are often protected from winds, and are in other ways provided with 

 special local habitat conditions characterized by greater humidity and 

 more shading, all of which induce more vigorous growth (Zinger, 

 1909). Thus, plants of a more or less xerophytic nature and growing 

 under xerophytic climatic conditions may, by cultivation, be trans- 

 formed into mesophytes. but these latter, when left uncultivated, 

 become entirely unviable. A similar phenomenon we observe in trees. 

 In a forest the trees have tall, slender trunks and weakly developed 

 crowns, while, when grown farther apart, they have short, sturdy 

 trunks and vigorous crowns. The latter easily resist winds, whereas 

 forest trees, when isolated, e.g., by the cutting out of trees, are often 

 blown down by heavy winds. 



5. Double flowers, induced by the conversion of stamens into petals, 

 constitute a clear example of an aberration rendering the affected 

 plants incapable of life without man's assistance, except, in certain 

 cases, through adequate vegetative means of propagation. 



6. Loss of various defensive adaptations, such as thorns, spines, pubes- 

 cence, etc., making the plant defenceless against herbivorous animals 

 and more susceptible to injury from evaporation of moisture. 



7. Exceptionally great increase in size of fruits, markedly distinguish- 

 ing cultivated from wild plants, may not be of advantage to the plant, 

 since it appears to render the fruits more difiicult for animals to eat 

 and, consequently, constitutes a hindrance to seed dissemination. 



8. Improvement in the flavor of fruits tends to cause animals to eat 

 them up completely, seeds and all. 



9. Seedless (parthenocarpic) .fruits, just as double flowers, render 

 the independent existence of a plant absolutely impossible. 



All the above-mentioned peculiarities, occurring also among wild 

 plants as abnormalities, are, by means of conscious or unconscious se- 

 lection, intensified and highly developed in the case of cultivated plants. 



10. Complete absence of fruiting in some cultivated plants — due to 

 atrophy of the sex organs (as, for instance, to a considerable degree, in 

 the sugar cane), to absence of pollinators in places of cultivation far 

 distant from the plant's native habitat (e.g.. Yucca species in Europe), 



