8] MODIFICATION AND DISTRIBUTIONS OF CROPS 21'] 



and, although commonly hereditary, nevertheless often the result 

 of environmental impress during the life of the individual. Seed- 

 lings rendered weak by competition with their fellows under other- 

 wise favourable conditions of cultivation may not so much as survive 

 on transplantation ; or again, thin and delicate ' shade leaves ' of 

 some trees are liable to shrivel on exposure. Though drastic, these 

 are examples of mere ontogenetic change during a single plant's 

 life-time. 



(4) Loss of adaptations for dispersal — or even for the initial act of 

 dissemination. The fruits of cultivated Flax and of the Opium 

 Poppy do not dehisce when ripe, whereas those of their wild relatives 

 do. Again, among weeds, the inflorescences of some noxious 

 Brome-grasses break up less effectively than those of their wild 

 counterparts, which also tend to have longer awns. 



(5) Loss of protective coverings and sturdiness — for example in 

 cereals whose fruits are deprived of the usual outer husks, and in 

 the pods of many cultivated members of the Pea family (Leguminosae) 

 which lack the fibrous lining characteristic of their wild relatives. 

 Presence of the fibrous lining also causes the valves to curl up and 

 thus helps dissemination of the seeds. The commonly lesser 

 development of fibrous tissue in crop plants is apparently connected 

 with their growth in close stands — often protected from winds and 

 under conditions of favourable humidity, nutrition, and shading, 

 which all tend to promote rapid growth. Similarly, in a dense 

 forest the trees usually have tall and slender trunks and weakly 

 developed crowns, so that individuals left isolated on removal of 

 their neighbours are liable to be blown down, whereas in the open 

 the same species tend to be far more sturdy. 



(6) Increase in size of seeds and fruits — usually accompanied by a 

 decrease in their number. This tends to reduce their chances of 

 dispersal while at the same time reducing the plants' opportunities 

 for propagation. Moreover, the production of unnecessarily large 

 seeds and fruits is wasteful so far as the plants' economy is concerned. 

 How much more economical are the fruits of Fireweeds than of 

 Pumpkins, and how much more successful as colonists are the 

 former plants ! 



(7) Improvement of flavour — of seeds and fruits, which is a com- 

 mon objective of cultivation, tends to cause animals to eat them 

 more voraciouslv and completely, and so militates against effective 

 dispersal. 



(8) Conversion of perennials into annuals — is common in the 



