R. S. DE ROPP 



383 



of lAA or NAA. To place these compounds in the category of carcino- 

 genic or mutagenic agents along with such substances as the nitrogen 

 mustards or methylcholanthrene, seems hardly justifiable at present. 



The biological status of habituated tissue has been studied by Camus 

 and Gautheret (10). Habituated tissue of scorzonera was grafted to 

 unaltered root tissue of this plant, on which it gave rise to a voluminous 

 neoplasm similar to that induced by crown-gall tumor tissue. These 

 investigators favor the view that habituated tissue occupies an inter- 

 mediate position between normal and crown-gall tumor tissue. 



We will next consider overgrowths of genetic origin. Such over- 

 growths were experimentally induced by Kostoff (26) by hybridizing 

 certain species o( Nicofiana, notably A^ langsdorfii W\l\\ N.glatica. On the 

 stems, roots, and leaves of these hybrids tumors developed either spon- 

 taneously or as a result of wounding. The overgrowths varied in appear- 

 ance from fasciations such as are seen on plants affected with witches'- 

 broom to outgrowths lacking any outwardly visible structure. Whitaker 

 (42) attributed the formation of these overgrowths to a cytoplasmic 

 disturbance occasioned by the introduction of the chromosome compli- 

 ment of A^. langsdorfii (used as pollen parent) into the cytoplasma of 

 N. glauca (used as seed parent). 



Tissue cultures of these overgrowths were made by White (44) who 

 proved that the callus was capable of making indefinite growth on a 

 medium containing 2 per cent sucrose, mineral salts, and yeast extract. 

 Like habituated tissue and crown-gall tumor tissue this callus is able to 

 grow without added auxin. Though it grew at first in the form of white, 

 undifferentiated tissue masses, it was later shown by White (43) and 

 Skoog (37) to be capable of differentiating roots, stems, and leaves under 

 certain environmental conditions. White (46) also showed, by grafting 

 fragments from cultures of this hybrid callus onto the stem of healthy 

 A^. glauca plants, that the hybrid tissue fragments grew under these 

 conditions as tumors, and apparently possessed the property of propagat- 

 ing their tumorous nature indefinitely. 



The experimental induction of overgrowths by a virus was accom- 

 plished by Black (i) in 1945 using the virus Aiireogemts magnivetja, 

 the causal agent of wound tumor disease. This virus produces a systemic 

 infection in a considerable number of unrelated plants into which it can 

 be introduced either by agallian leaf hoppers or by grafting. Insects 

 infected with the virus were found to become infectious only after an 



