WHITE— STUDIES OF INHERITANCE IN PISUM. 495 . 



work carried on at the Brooklyn Botanic Garden consists in giving 

 the commercial stock of a variety as received a number such as 

 Pisum 12; the plants grown from seed of P12 are numbered P12-1, 

 P12-2, P12-3, etc. The next generation of P12-1 being recorded 

 as P12-1-1, etc. Crosses are designated thus: P12-1 X Pi4-i» the 

 Fi progeny as (P12-1 X Pi4-i)-i» -2, -3> etc. In F. and subse- 

 quent generations all seeds planted are counted, and plot sketches 

 kept of the arrangement of the plantings. A printed description 

 blank covering all the common characters of peas is used for records 

 of individual plants, and less detailed blanks for cultures studied 

 for special characters. 



Harvesting. — In studying seed characters extreme care should be 

 taken to allow proper conditions for maturity. Harvesting imma- 

 ture peas may lead to considerable errors in studies on cotyledon 

 and seed-coat color. Pea vines may be allowed to mature until no 

 green remains and they are dry and brittle. This insures maturity. 

 In order to avoid breaking such brittle material, the vines should be 

 thoroughly wetted with a hose before handling. Each plant should 

 be labeled with a tag label as gathered. Green cotyledon varieties 

 tend to fade to yellow if left exposed to light for a considerable 

 time (54) and damp wet weather at harvest time may bring about 

 the same result much sooner (i,'6o, 21). 



Environment. — No factor is of more importance in a detailed 

 genetic study of the characters of a group of plants such as Pisimi, 

 than environment. Environment, being the co-partner of heredity 

 in the make-up of a character, should have just as precise a descrip- 

 tion as the characters themselves, or else be eliminated altogether 

 by growing the material under as near as possible the same condi- 

 tions. If environment were as easy a proposition to handle as in 

 the case of chemical experiments, one could define it in the case of 

 each experiment with such exactness that it could be easily repro- 

 duced. Unfortunately this is not practicable, because of the many 

 factors which compose it. Under greenhouse conditions, it is more 

 practicable than in field cultures. However, even here, aside from 

 the temperature, watering, etc., factors such as soil and light vary 

 so over an area when large cultures are grown, that it is largely a 

 figure of speech to speak of absolute uniform environment for the 

 whole area. 



