THE PROPAGATION OF PLANTS 133 



99. Conditions which favor the Entrance and Growth 

 of Spores. — (1) Unhealthy Condition of the Plant which 

 they attack. — Plants that are weakened by drouth or by 

 the attacks of insects, or those that have become shaded 

 by weeds are more Uable to be destroyed by disease 

 than others. The disease, in other words, makes the 

 greatest progress when the plant is in an unhealthy 

 condition. 



(2) Moisture. — Moisture favors the growth of spores. 

 In low, damp places, in dark, wet cellars, where fruit and 

 vegetables are stored, the spores of disease and decay 

 thrive. In Section 43 we learned that the greatest enemy 

 of stored seeds is moisture. This is partly due to the fact 

 that it favors the growth of molds which may eventually 

 cause the death of the embryo. 



(3) Darkness. — Direct sunlight will do much to prevent 

 the propagation of spores. We therefore destroy weeds, 

 leave sufficient space between our plants, and prune our 

 trees to let in the sunUght. 



(4) Heat. — Spores spread and grow best in warm 

 places. This is why we put fruit and vegetables in cool 

 cellars, in cold storage or in the refrigerator. 



(5) Careless Treatment. — By bruising or breaking the 

 skin of fruit or vegetables, we may open the way for the 

 entrance of spores, and decay will follow. Similarly, by 

 careless pruning or cultivating, we may allow the spores 

 of disease to gain entrance into Hving plants. 



100. Propagation by Seed. — The higher plants are 

 commonly propagated by seed. However, this means of 

 propagation will be considered in detail in the chapters on 

 special crops. In Chapters VII and VIII, we have already 

 learned essential facts concerning selection, storing and 

 germination of seeds. 



