296 Non-Isolablc Races. 



regions as pure biennials, in others as annuals, and in 

 still others in a mixture of these two forms. ^ 



Inasmuch therefore as the biennial habit is to be re- 

 garded as the character of the species and the annual 

 habit as the anomaly, the latter is likely to follow the 

 general rule according to which the development of the 

 anomaly is favored by improved conditions of life. And 

 the experiments which I propose to describe in this sec- 

 tion prove the correctness of this view. 



However, there is an apparent contradiction, for, as 

 is well known, Rimpau has shown in the case of the 

 beet that every retardation or interruption of the growth, 

 whether it occurs during germination or just after the 

 seed comes up or at a later stage of the development of 

 the plant, favors the production of the seed in the first 

 year of the plant's life.^ 



But in this case it only appears that we are dealing 

 with conditions favorable to the production of the anom- 

 aly whereas in reality we are concerned with the stimulus 

 necessary for the manifestation of this bolting. As it 

 is not very easy to make this difference clear I shall select 

 an instance of a pure biennial race^ which lacks the power 

 of giving rise to annual specimens. I refer to my cul- 

 tures of Dipsacus sylvestris. This race can be sown at 

 any period of the year, and the plants will always remain 

 rosettes until the end of the next winter and develop a 

 stem in the spring of their second year. According to 

 whether the sowing was made in the spring or in the 

 summer or not till autumn are the rosettes vigorous or 



^ Instances of this are given by J Costantin, Les vegetaux et 

 les milieux cosmiques, Paris, 1898, pp. 28 f. 



^ Landw. Jahrbiicher, passim, 1880, p. 194. 



' On Biastrepsis and Its Relation to Cultivation, Annals of Bot- 

 any, Vol. XIII, No. LI, Sept. 1899, p. 395. 



