VEGETATIVE PROPAGATION 215 



state, e.g. Poa alpina, Deschampsia caespito.sa, and particularly the 

 mountain variety' of the Sheep's Fescue {Festuca ovina, var. vivipara). 

 Such developments may be held as a response to stress of circum- 

 stances. The short season and alpine conditions being unfavourable 

 for flowering and fruiting so as to set seed, the formation of vegeta- 

 tive buds gives a greater certainty of survival. Any advantage 

 that follows from sexual propagation is sacrificed to attain that 

 certainty. 



Such examples suggest how various are the ways in which Flowering 

 Plants propagate by budding. It is, however, rare among Gymno- 

 sperms. In lower forms, such as Ferns, Horse-tails, and Club-mosses, 

 it is common. Among the most prolific of all plants in this way are 

 the Mosses, some of which are practically unknown in fruit. Finally 

 in the Algae, and especially in the Fungi vegetative propagation is 

 conspicuous as a source of increase. This raises the question whether 

 this method might not suffice for all practical purposes. Cases are 

 known among cultivated Flowering Plants where it is continued 

 indefinitely. The cultivated Banana and the Pineapple are seedless. 

 The Sugar Cane rarely flowers. The Jerusalem Artichoke has been 

 grown regularly in British gardens for two centuries from tubers. 

 There appears in fact to be no definite limit to repetition of increase 

 by budding. It has always been favoured by horticulturalists for 

 the good reason that the exact qualities of the strain or variety are 

 retained, while in propagation by seed those qualities are liable to 

 be modified or lost. It is the weakness, as it is also the strength of 

 somatic budding, that there is as a rule no evolutionary change, but only 

 repetition. 



This repetition is exactly what the horticulturalist requires when 

 dealing with special strains of cultivated plants. His methods depend 

 on the maintenance of the desired strain through buds produced in 

 normal sequence, or adventitiously induced. Cultiijgs and slips are 

 merely parts of the shoot of the parent plant bearing one or more 

 normal buds. They are kept under circumstances to promote root- 

 formation, which takes place best if the cut be made just below a 

 node. The shoots must be selected of the right age and condition 

 to secure success. Greater certainty follows on layering, in which 

 the shoot it is desired to establish is not separated from the parent, 

 but pegged down in the soil, with or without a notch or ring, cut so 

 as to check the downward flow of plastic material. This promotes 

 root-formation, after which the shoot may be detached (Fig. 163). 

 This method is used for rapid production of established plants in the 



