WILLIAM J. ROBBINS 467 



media containing mineral salts and sugar. On this medium the stems 

 grew actively through repeated transfers extending for 22 months. In 

 the dark, however, growth approached zero after a few transfers. The 

 growth requirements for excised stem tips cultivated in the dark are 

 still obscure. The slow absorption of sugar and other materials by the 

 excised stem may be an important factor. 



Excised Tissues of Stems or Storage Organs. — Nobecourt, Gautheret, 

 and others have obtained unlimited growth of portions of stems, storage 

 organs, or callus of a considerable number of species of plants. These 

 include among others: carrot {Daucus carota)\ chicory [Cichorium 

 Intybus); Cissus discolor; grape {Vitis incisa, V. vinifera var. Aramon., 

 V. Coignetiae, V. Davidii); hawthorne {Crataegus monogyna); Jerusalem 

 artichoke {Helianthus tuberosus)\ Parthenocissus tricuspidata; P. Hederae- 

 folia; Rubus fructicosus; Scorzonera Hispanica; snapdragon {Antirrhinum 

 majus); and turnip {Brassica campestris) (18). 



Indoleacetic acid or its equivalent is of primary importance for the 

 growth of these tissues. It represents an essential factor for many of them. 

 Some (carrot, grape, Jerusalem artichoke, salsify {Tragopogon), Scorzo- 

 nera, and turnip) grow slowly without the addition of lAA to the medium 

 (17,18), but their growth is much more rapid when the medium is 

 supplemented with this substance. Apparently they synthesize a small 

 amount of lAA but insufficient for maximum growth. Cysteine and 

 thiamin improve the growth of some of these tissues; the callus of 

 Salix caprea and the stem tissues of hawthorne {Crataegus monogynd) 

 are reported to require pantothenic acid and biotin in addition to lAA 

 (18,19). 



The relation of lAA to the growth of excised tissues of stems and 

 storage organs suggests that this substance or its equivalent acts as an 

 essential metabolite which is not synthesized in adequate quantities by 

 the isolated tissues of stems and storage organs. Its relation to these tissues 

 appears to be of much the same order as the relation of thiamin, pyri- 

 doxine, or niacin to the growth of excised roots. 



Plant Embryos. — Considerable attention has been devoted to the 

 culture of plant embryos by Blakeslee (2), Hannig (20), LaRue (26), 

 Tukey (55), van Overbeek (56), and others. Hannig more than 40 

 years ago cultivated embryos of some of the Cruciferae. Immature em- 

 bryos evidenced little development after removal from the seed; older 

 ones were grown to maturity. In general, attention has been devoted 



