156 



Variations 



but so far as is at present known, they are rare. The occur- 

 rence of double flowers in species with usually single flowers, 

 of regular flowers in species with usually irregular flowers, 

 of divided leaves in species with entire leaves, of a weeping, 

 or fastigiate, habit in trees and shrubs with an erect, or spread- 

 ing, crown, respectively, are among the most common examples 

 of mutations at present known. 



Monstrosities. Very striking variations which give the impression of an 

 altogether abnormal structure are called monstrosities. They are 

 often termed malformations and frequently give rise to diseased 

 conditions. They are of rare occurrence in nature ; the 

 cauliflowers and turnips of cultivation are believed to have 

 arisen from monstrosities. Finally it must be noted that 

 remar kable variations in the form of plants may be caused by 



Mutilation. insects and fungi, which may, for instance, be responsible 

 for the peculiar structures known as " galls " and " witches' 

 brooms." Mutilation also may exercise a far-reaching effect 

 such as is seen in plants which are continually browsed cr 

 grazed by animals, while some annual species may be made 

 perennial by removing the flower buds and thus preventing 

 their reproduction. 



Heredity. 138. By heredity is understood the 



transmission of characters from a parent to its offspring. 

 This transmission is rendered possible in plants by means 

 of their powers of sexual and asexual reproduction. Many 

 variations can only be transmitted truly to the offspring 

 by asexua propagation, and for this reason individuals 

 which exhibit a desirable variation are usually propagated 

 in horticulture by asexual methods. This is particularly the 

 case with hybrids which are frequently very inconstant if 

 propagated by seed. The younger a plant or organ is, the greater 

 will be the effect of any factor which is capable of influencing its 

 development, and the early stages of the growth of the embryo 

 constitute the most susceptible period. If this period could 

 be cut out, as it were, from a plant's life history we should 

 expect it to be less subject to variation and to resemble 

 its parent more closely than would be the case if the individual 

 were raised from seed, and this is precisely what is effected by 

 asexual propagation. It must, however, never be forgotten 

 that in sexual reproduction effected by the crossing of two 

 distinct individuals a portion of the protoplasm of each parent 

 enters into the composition of the young plant, which must 

 therefore inherit something from its father and something from 



