PLANT-BREEDING METHODS 421 
hands off. Or a strong wind may succeed in forcing into the greenhouse 
seeds of the same species as the one undergoing investigation. Or pests 
Fig. 171.—Pedigree cultures of Bursa in greenhouse of the Department of Botany, Prince- 
ton University. (Photo from G. H. Shull.) 
of various sorts may appear—damping off, insect pests, snails, slugs, mice, 
and in the breeding garden birds, gophers, moles, rabbits, ete. In this 
connection it may be sufficient to say that eternal vigilance is the price 
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