376 Plant Physiology 



grown from seed for several centuries. The European 

 grape-vine ( Vitis vinifera) has been grown more than 5000 

 years, and vegetative propagation has been the rule. 

 When native American vines were carried to Europe, 

 diseases new to Europe were introduced, and European 

 vines were so susceptible that enormous injury resulted. 

 Many thought that this weakness with respect to disease 

 was a result of the long vegetative culture ; but this seemed 

 to be disproved by the fact that seedling sorts of that 

 species of vine were equally susceptible. 



Resistance or susceptibility to any disease appears to be, 

 in many cases, a character, or a complex of characters, and 

 may follow known laws of heredity, as in the case of other 

 characters subsequently discussed. Moreover, among the 

 more familiar molds and other fungi there are some notably 

 ubiquitous and vigorous forms which are not known to 

 possess sexual stages. Among the species of living things 

 generally, however, the frequency of gametic fusion, on 

 the one hand, and the complete loss of this process among 

 others constitutes a biological paradox. 



225. Relation of vegetation to fruiting. Plants exhibit 

 a remarkable diversity in the relations between vegetative 

 development and fruiting. With respect to annual, 

 biennial, and perennial habits this has been briefly con- 

 sidered. Generally speaking, fruiting is the climax of a 

 continuous or interrupted period of vegetative develop- 

 ment. The American Agave grows many years vegeta- 

 tively, and then through the formation of an enormous 

 flower-stalk and abundant fruits the leafy parts are drawn 

 upon to such extent that they are left exhausted and 

 incapable of recovery. 



