Regeneration 251 



these appear only when the leaf is removed from the plant or its vigor 

 reduced. Instead of actual plantlets, bulbils or bulblets may be formed, 

 modified buds which drop from the plant and produce new individuals. 

 Many plantlets develop from preformed meristematic cells or cell 

 aggregates and thus are clearly to be regarded as reproductive struc- 

 tures even though in some cases they are induced only by rather ab- 

 normal conditions. Others arise from unspecialized cells, usually epi- 

 dermal or subepidermal ones, much as do the shoots on the hypocotyls 

 previously described. When these are frequently formed in nature they 

 are usually to be regarded as reproductive rather than regenerative struc- 

 tures. Only a few typical examples can be mentioned here. 



Fig. 9-8. Leaves of Achimenes, used as cuttings, regenerating roots and bulbils from 

 the base and producing plantlets where veins have been cut. ( From Goebel. ) 



A familiar one is that of Tolmiea menziesii (Yarbrough, 1936a), in 

 which a plantlet regularly is formed at the junction of petiole and blade 

 from a preformed bud at that point. This readily separates from the 

 parent plant and forms a new one. In Cardamine pratensis (Goebel, 1908) 

 adventitious shoots or plantlets grow in the autumn or under special 

 conditions from the axils of the leaflets by the activity of groups of 

 meristematic cells. At the junctions of the larger veins occur slight 

 swellings and these may also develop into plantlets (Fig. 9-9). In such 

 forms there are evidently many cells that can easily be induced to become 

 meristematic and form plantlets. How such cells differ physiologically 

 from others it is important to discover. 



