WHITE— STUDIES OF INHERITANCE IN PISUM. 493 



are special reasons for planting all the seed. The seed are all 

 counted, so that any distortion of ratios from this source can be 

 checked up. Wire netting may be used to keep the tall varieties off 

 the ground. Peas should not be planted on the same ground two 

 successive years, mainly on account of increased liability to pea 

 diseases the second year. Darbishire (21) planted his pea plots to 

 vetch for two years before using them again. Peas may be grown 

 successfully in four-inch (10 cm.) pots or in benches in the green- 

 house during the winter months. A bamboo stick or string should 

 be provided for each greenhouse-grown plant. The greenhouse 

 temperature should not be higher than 45°-55° F. Higher tempera- 

 tures promote trouble with red spider and with various pea diseases. 



Crossing. — Crossing in peas is easily accomplished by the re- 

 moval of the stamens from a half-grown bud and the immediate 

 application of the foreign pollen to the stigma. Pollen may retain 

 its viability in a dry Petri dish for a week or more. Tschermak 

 (81) made successful crosses with 14-21 days' old pollen of 

 Allerfriiheste Mai. Varieties such as Dwarf Gray Sugar and other 

 early-blooming sorts discharge their pollen while the bud is still 

 greenish, while in many of the late-flowering sorts, the flowers are 

 nearly mature before self-pollination takes place (78). Mutilation 

 of the flower rarely causes the flower to fall, and if the crossing is 

 done during sunny weather, most of the crosses will be successful. 

 Under greenhouse conditions, peas have scattering flowers even 

 after the first crop of pods are ripe. These scattering flowers may 

 be utilized to furnish pollen for crosses with late-flowering forms. 

 In field plots, crossed flowers should be protected by square-bottom 

 paper bags. In greenhouse cultures, this precaution is generally 

 unnecessary, especially in winter. Diluted grain alcohol is used to 

 kill stray pollen on hands and the instruments after each cross. 

 Usually the pollen to be applied is carried on the stigma and this 

 foreign stigma brushed across that of the flower to be crossed. In 

 labeling the cross, each plant of a variety is given an individual num- 

 ber, and care is taken so that each plant used in crossing also bears 

 several uncrossed pods. The maternal parent is designated first. 



Sclf-fertUization. — Because peas are naturally self-fertilizing, 

 protection of the flowers of both pure strains and hybrid generations 



