4 PLANT PROPAGATION 



centuries (big trees, giant redwood of California and cer- 

 tain species of oak and pine). Under normal (or natural) 

 conditions, the duration of the life cycle of any species 

 may vary considerably because perhaps of inherited vigor 

 or environment or both. 



For instance, in a sowing of garden carrots a few plants may "run 

 to seed" the first season, though the general life cycle of this veg- 

 etable is two years ; conversely, some annuals, as radish, may fail to 

 seed the first year, but send up flower stalks the following season. 

 Such cases are, however, exceptional. 



So far as known, no plant lives indefinitely, though by 

 the application of certain methods of propagation exist- 

 ence may be continued beyond the duration of the normal 

 life cycle of the plant so treated. 



For instance, the geranium, which is normally a warm climate 

 plant, easily killed in cold climates by frost, may be propagated by 

 means of cuttings, and thus not only its numbers increased indef- 

 initely, but its life thus extended by asexual generation. In one 

 sense this is not strictly extending the life cycle of the individual 

 plant, for the original stem and roots are generally thrown away as 

 having served their purpose. 



Because all plants normally reach the limits of their 

 life cycles, some method of propagation is necessary if 

 they are to be perpetuated ; otherwise they will be lost. 

 To prevent this contingency among flowering plants, 

 nature usually provides ample seed, though in some cases 

 asexual methods have been developed. Strawberries 

 propagate by means of runners (102) ; certain dogwoods 

 by stolons (123) ; black raspberries by layers (96) ; 

 houseleeks by rosettes (126) ; cannas by rhizomes (122) ; 

 banyan trees by aerial roots from limbs; mangroves by 

 their "knees" or prop roots; Irish potatoes by tubers 

 (128) ; and so on. 



6. The term environment is used to include all the ex- 

 ternal influences that, as a whole, affect a living organism 

 in any way. Among the principal factors that make up 

 environment are heat, light, moisture, and food supply. 



7. Duration of life cycle determines the three general 

 groups of plants ; annuals, biennials and perennials. 



