4^6 PLANT GROWTH SUBSTANCES 



There is, however, a substantial number of species for which the 

 conditions for unUmited growth of excised roots are unknown. Among 

 these are included: Bauhinia purpurea; beet {Beta vulgaris)', broccoH 

 {Brassica oleracea); Bryophyllum calydnum; cabbage {Brassica oleraced)\ 

 corn [Zea Mays); cucumber {Cucumis sativus); eggplant {Solanum 

 Melongena); grape {Viiis sp.); grapefruit {Citrus maxima); kohlrabi 

 {Brassica oleracea); lemon {Citrus Limonia); lettuce {Lactuca scariola); 

 lupine {Lupinus sp.); orange {Citrus sinensis); Parthenium argentatum, 

 Poa sp.; Poinciana gilesii; potato {Solanum tuberosum); rice {Oryza 

 sativci); Simmondsia californica; Sterculia diversifolia; Thuja orientalis; 

 3 species of tobacco {Nicotiana glutinosa, N. rustica, N. sylvestris); and 

 Wisteria sinensis (6,10,60). Potentially unlimited growth has not been 

 obtained for the roots of any monocot or cucurbit, and the roots of 

 many woody plants have proved to be refractory. 



Other conditions than vitamin requirements may be important in 

 determining unhmited growth of excised roots. This is suggested by the 

 peculiar results obtained by Robbins and Maneval (45) with lupine, 

 and by Bonner (6) with the roots of Stercidia diversifolia, Bauhinia 

 purpurea, and Wisteria sinensis. In these cases out of many roots the 

 majority failed to grow under excised conditions, but a single individual 

 might show very substantial growth. McClary (31) has reported that he 

 could obtain unlimited growth for the excised roots of a hybrid corn 

 on an agar medium containing sugar and mineral salts and has suggested 

 that physical factors are determinative rather than vitamin supplements 

 or other growth substances. Bonner was unable to confirm McClary 's 

 observations on the unlimited growth of corn roots, and we have not 

 been successful in obtaining unlimited growth. 



Excised Stem Tips. — Efforts to cultivate excised stem tips are compli- 

 cated because, as a rule, the stem tips develop roots and the investigator 

 is then dealing with an entire plant. Dodder and asparagus stem tips are 

 relatively free from this complication. Loo (29) found that seedlings 

 of dodder grew well during the first week but ceased to grow in the third 

 week in a sucrose-mineral salt solution in diffuse light. Stem tips of 

 dodder weie kept alive in diffuse light in a sucrose-yeast extract medium 

 for 10 months through a series of transfers. The stem tips developed 

 considerable chlorophyll. In the dark, however, they failed to maintain 

 their growth even in the sucrose-yeast extract medium. Loo (27,28) 

 also cultivated isolated stem tips of asparagus in diffused light on nutrient 



