CHAPTER IV. — SPECIAL PHYSIOLOGY OF REPRODUCTION. 777 



• being stimnlated to produce seeds without any sexual process, that 

 is, parthenogenetically {p seud aga my). 



Somewhat analogous to these phenomena are those presented by the so- 

 called graft-hyhrlds (see p. 768), in which scion and stock seem to mutually 

 afifect each other. For instance, the Bizzaria-Orange is a tree which bears 

 branches and fruits characteristic of Citrus Auraiitium and C. medico, and of an 

 intermediate form, and is stated to be a graft hybrid of these two species; 

 similarly Cytisns Adami is a plant possessing characters intermediate between 

 those of Cytisus Lalmrnuin and C. purpureas, and is stated to be a graft-hybrid 

 of C. purpureus on C. Laburnum ; but since the evidence that these plants 

 really are graft-hybrids is inconclusive, it is unnecessary to consider them 

 further. 



§ 22. Heredity and Variation. The term heredity is used 

 to express the fact that an organism inherits in some degree the 

 characters of its parent or parents. The degree of heredity — that 

 is the degree of resemblance between ofPspringand parents — varies 

 widely, inasmuch as not all the characteristics of the parent are 

 hereditary in all modes of reproduction. The highest degree of 



[resemblance is attained by means of vegetative propagation : by 

 this means the [)eculiar features of the individual may be exactly 

 reproduced in the offspring, and consequently it is extensively 

 made use of in horticulture (e.g. propagation by cuttings and 

 bulbs ; budding of Roses ; grafting of fruit-trees, and of ornamental 

 trees and shrubs with variegated or otherwise abnormal foliage, 

 etc.) to perpetuate some specially valuable peculiarity of a plant, 

 and to multiply the number of individuals possessing it. Such a 

 high degree of resemblance between parents and offspring cannot be 

 attained by means of sexual reproduction, for many of the peculiar 

 characteristics of the parents are not transmitted by this means. 



J The plants resulting from a sexual process between two individuals 

 of the same variety or species present, not so much the character- 



' istics of the parents, as the characters of the variety or species. 



I The phenomena of heredity in connexion with sexual reproduc- 

 tion are especially striking in the case of hybrids, though the 

 result of the crossing of species or varieties, as shown by the 

 characters of the progeny, is by no means always the same. 

 Whilst it is true, as a rule, that hybrids are intermediate in 

 character between the species or varieties to which their parents 

 respectively belong, this is by no means always the case ; for some 

 hybrids resemble the one parent more than the other, either in 

 external form and colour, or in properties such as resistance to 



