358 TEXTBOOK OF PLANT PHYSIOLOGY 



of the sexual process. There is a common belief that in case of 

 asexual reproduction, the type or clone, will finally degenerate and 

 that the sexual process must be employed in order to restore it. 

 This idea, however, has not been definitely proven. Many 

 instances of a complete loss of sexual reproduction are known in 

 higher plants, such as yams, bananas or onions and yet such plants 

 do not show any symptoms of degeneration. This is still more true 

 with lower plants, fungi in particular, many of them having lost 

 almost all traces of sexual reproduction. 



Vegetative propagation, which is in fact merely a separation 

 of certain parts of the parent plant, has the advantage of not 

 requiring many of the frequently very complicated steps for secur- 

 ing a progeny (the transfer of pollen, etc.). This process, therefore, 

 assures with greater certainty an uninterrupted existence of a 

 species. 



104. Propagation by Cuttings and Layers. Regeneration and 

 Polarity. — Owing to a high degree of physiological independence 

 which is characteristic of almost all parts of a plant and even of 

 its separate cells, the latter have to a considerable extent the 

 capacity for restoring all the lacking organs and of producing a 

 completely new individual. This knowledge has been extensively 

 utilized in agriculture, but particularly in horticulture. Since 

 ancient times, a large number of methods for the artificial propa- 

 gation of plants have been in practical use. By artificial propa- 

 gation is meant such methods which do not require the use of any 

 special organs of sexual or asexual reproduction formed by the 

 plant itself, but which utilize any convenient and suitable vegeta- 

 tive part. 



The most popular method of artificial propagation is by the 

 use of cuttings and layers. By a cutting is meant any part of 

 either the stem, the root, or even the leaf of a plant, which, having 

 been separated from the plant and placed in favorable conditions 

 for its development, is capable of growing into a new organism. 

 If such a vegetative part is left organically connected with its 

 maternal tissues, though only during the first stages of rooting and 

 growth, then it is called a layer. Both of these methods are very 

 popular among fruit growers. Layering is used in cases when cut- 

 tings do not root readily, as, for instance, in the case of grapes, 

 hazelnuts, mulberries, gooseberries, etc. In order to obtain a 

 layer, a branch is usually bent down and covered with soil for some 



