HYBRID VIGOR OR HETEROSIS 155 



and Gartner 74 gives them his especial attention. Under 

 the heading, "Ausdauer und Lebenstenacitat der Bas- 

 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 of the same species have 

 a longer life at times as, for example, Draba verna which has both 

 annual and biennial forms. The longevity of a plant thereby furnishes 

 no specific difference 1 but at most only signifies a variability. However, 

 in hybrids this difference deserves special consideration. In most 

 hybrids an increased longevity and greater endurance can, be observed 

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

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

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

 coming hybrid, as the union of Hyoscyaiwus agrestis with niger, Nicotiana 

 rustiea with perennis, Calceolaria plantaginea with rugosa shows. So 

 also in reciprocal crosses when the perennial species furnishes the seed 

 and the annual species supplies the pollen, as Nicotiana glauca with 

 Langsdorffii, Dianthus caryophyllus with chinensis; Malva sylvestris 

 with mauritiana or biennials 1 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 which 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 Kolreuter 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. Gernert 77 states that teo- 

 sinte and the first generation cross of teosinte and maize 



