Regeneration 253 



at the start of shoot development ( strong protoplasmic streaming, migra- 

 tion of the nucleus, and increase in cytoplasm). Here and in a good 

 many other plants, shoots will appear on the blade if one or more of the 

 veins are cut (Fig. 9-10), thus perhaps preventing the access of in- 

 hibiting substances. Prevot was able to induce bud formation on begonia 

 leaves by the application of various substances and by growing the 

 plants in the absence of oxygen. He also found that strips of epidermis 

 removed from the leaf would form buds. Not all begonias have high re- 

 generative ability, and this seems to be an inherited character when 

 various types are crossed. 



Much like these cases is the development of young plants on the leaves 

 of Saintpaulia ionantha. This species is often reproduced in cultivation 

 by plantlets formed on leaves that have been cut off and placed in a 

 humid atmosphere. From individual cells of the upper epidermis shoots 



Fig. 9-10. Propagation of rex begonia. If a leaf is removed and placed on moist sand 

 and certain of its major veins severed, plantlets will regenerate at these cuts. (From 

 Avery and Johnson.) 



develop, and roots originate from parenchymatous cells near the boundary 

 between xylem and phloem in the veins ( Naylor and Johnson, 1937; Fig. 

 9-11). 



Torenia asiatica behaves in much the same way (Winkler, 1903). In 

 leaf cuttings, numerous shoot primordia begin to develop over the sur- 

 face of the blade, each from an epidermal cell above a vein. Only a few 

 of these primordia grow into shoots, and a single leaf thus shows a 

 wide range of stages in shoot development. The shoots that form come 

 to flowering very quickly, sometimes when they have only one well- 

 developed leaf, and should thus make excellent material for a study of 

 the factors that induce flowering. 



Many members of the family Gesneriaceae, to which these plants be- 

 long, regenerate readily. Leaf cuttings of Achimenes produce clusters of 

 bulblets at the base of the petiole and shoots from the blade if the veins 

 are severed ( Doposcheg-Uhlar, 1911). Streptocarpus (Goebel, 1908) illus- 

 trates most types of regeneration. 



