[Chap. XXXVII HEREDITY IN PLANTS 441 



leaves, stems, and roots of a plant usually have the same hereditary 

 properties, or potentialities. 



Properties, or potentialities, are merely the qualities or attributes of 

 material objects and of their special arrangements in organized systems, 

 or mechanisms. We may infer therefore that the individual cells of the 

 several vegetative organs of a plant have within them exacth' the same 

 structural mechanism of inheritance. Furthemiore, since cells multiph' 

 bv cell division (two cells being formed from one), this hereditary 

 mechanism must be exactly dupHcated by some means every time cells 

 divide. 



This intrinsic mechanism by which the hereditary potentialities of the 

 cells of the vegetative organs of a plant are preserved throughout its 

 lifetime is surprisingly stable. In long-lived plants such as the redwood, 

 and in plants that propagate vegetatively this mechanism may be per- 

 petuated unaltered for centuries. Occasionally it becomes altered in 

 some of the branches of a plant. Stable alterations in this mechanism are 

 heritable, and are the means by which new kinds of plants are deri\'ed 

 from preexisting ones. Seedless oranges and the red-rooted sweet potato, 

 Plate 4, are examples of heritable variations that occurred in certain 

 l:)ranches of the plant. Heritable variations that occur in this manner are 

 known as mutations. 



By cross-fertilization certain parts of the hereditary mechanism of the 

 two parent plants are brought together in the fertilized egg. That is, the 

 fertilized egg contains the hereditary mechanism that was in the sperm 

 and also the one that was in the egg. The plant that dexelops from this 

 fertilized egg thus possesses a hereditary mechanism partly like and 

 partlv unlike that of either parent. In certain characters it may resemble 

 one or the other parent, while in other characters it may differ from 

 both parents. Such individuals are called hybrids. When hybrids repro- 

 duce by seeds, their progeny usually are variable in appearance. 



For example, when cross-fertilization occurs between red-flowered 

 and white-flowered plants of snapdragon, the resultant progeny are 

 hybrids and all of them are pink-flowered. This difference between 

 parents and progeny is a hereditary difference. Cross-fertilization be- 

 tween the pink-flowered plants, or self-fertilization within any one of 

 them, results in progeny in which approximately one-fourth of the 

 plants are red-flowered like the original red-flowered plant, two-fourths 

 are pink-flowered like the immediate hybrid parent, and one-fourth are 

 white-flowered (Fig. 205).* 



