Abnormal Growth 



297 



they consisted of only one kind of cells, as in many animal cultures. 

 Most were really organ cultures. Those grown from root tips can be 

 carried through an indefinite number of subcultures and produce large 

 masses of root tissue. Shoot axes may be cultured in the same way 

 from apical meristems, and Nitsch (1951) and others have succeeded 

 in growing fruits from small primordia. Leaves can be grown to maturity 

 in the same way (Steeves and Sussex, 1957), as also can ovules 

 (Maheshwari, 1958). These organ cultures have given much informa- 

 tion on the nutritional requirements of various parts of the plant and have 



Fig. 11-13. Culture of stem callus of tobacco on nutrient agar, six weeks after trans- 

 fer to new medium. (Courtesy Department of Plant Pathology, University of Wis- 

 consin. ) 



been of importance for an understanding of their physiology. Wet- 

 more (p. 222) grew fern plants from shoot apices in culture and found 

 pronounced morphological effects of differences in the medium, a re- 

 sult of much morphogenetic significance. 



Something closer to a true culture can be attained by growing calluses, 

 tumors, parenchyma, and bits of tissue from the cambial region (Fig. 

 11-13). Structures much like amorphous tumors and galls result. In these 

 the strict morphogenetic control is relaxed and the explanted material 

 may be grown in unlimited quantities by subculturing. Such cultures 

 have been made of spontaneous tumors and of secondary crown gall. 



