HYBRID VIGOR OR HETEROSIS 155 



and Gartner "^^ gives them bis especial attention. Under 

 the heading, "Ausdauer und Lebenstouacitiit der P^as- 

 tardpflanzen, ' * he makes the following statements: 



There is certainly no essential difference between annual and 

 biennial plants and between these and perennials in regard to their 

 longevity, for frequently different individuals oi' the same species have 

 a longer life at times as, for example, Draba venia which has bolh 

 annual and biennial forms. The longevity of a plant thei-eby f umi^hea 

 no specific difference but at most only signitictj a variabihty. ILowever, 

 in hybrids this difference deserves special consideration. In mo^t 

 hybrids an increased longevity and greater endurance can bo obeerved 

 as compared to their parental races even if they come into bloom a 

 year earlier. The union of an annual, herbaceous female plant uith a 

 perennial, shi-ubby species does not shorten the life cycle of the forth- 

 coming hybrid, as the union of Hyoscyanms agrestis with niger, Nicotiunn 

 rustica "with perennis, Calceolaria plantaginea with rugosa shows. So 

 also in reciprocal crosses w^hen the perennial species furnishes the seed 

 and the annual species supplies the pollen, as Nicotiana glauca v^ilh 

 Langsdorfjiij Dianthus caryophyllus with chitiensis, Malva sylcestns 

 with mauritiana or biennials with perennials and reciprocally, as 

 Digitalis purpurea with ochroleuca or lutea, and lutea with purpurea, or 

 ochroleuca with purpurea. From, the union of two races of different 

 longevity a hybrid usually results into Avhich the longer life of one or 

 the other of its parent races is carried whether it comes from the male 

 or female parent species. 



Many more instances are given by Gartner supporting 

 the conclusion previously reached by Kolr enter that the 

 longer life of hybrid plants is to be counted among their 

 usual properties. 



Gartner also gives several examples of endurance to 

 unfavorable weather conditions by hybrids. Many of his 

 tobacco hybrids actually survived the winters in the open 

 field in south Germany when the parents were killed. 



The hardiness of hybrids is frequently shown by a 

 great resistance to parasitism. Gornert'^'^ states that teo- 

 sinte and the first generation cross of teosinte and maize 



