124 SINNOTT 



happens, for example, in the formation of roots on decapitated roots, or ad- 

 ventitious buds on decapitated shoots, or roots on stem cultures, and in 

 similar cases. In such instances important information can be gained as to 

 the physiological basis of regeneration, such as its relation to polarity, gravity, 

 and growth substances. 



Tissue regeneration in plants often provides clues impossible to find in 

 normal development. In the regeneration of a vascular strand after it has 

 been cut, for example, a new bundle is often formed by the transformation 

 of large cells that would normally be parenchymatous. In such a cell cyto- 

 plasmic granules can be seen to aggregate into a pattern which marks the 

 future reticulate lignified wall. Since the problem of differentiation, as is 

 often remarked, must be studied in the cytoplasm because the nucleus is the 

 same in every cell, these examples of tissue regeneration provide good mate- 

 rial in which to investigate the behavior of the cytoplasm on a much larger 

 scale, so to speak, than in normal tissue. 



Orderly development sometimes breaks down, in plants as in animals, be- 

 cause of various internal or external factors. These cases of atypical growth 

 are useful for a study of the developmental process itself. Most plant galls 

 are self-limiting and rarely, if ever, assume a type of growth that can be 

 called malignant. The intensive work on crown gall, however, has provided 

 some important clues for a comparison between galls and true cancers. 



The great variety of abnormalities that are to be found show the remark- 

 able possibilities that plants possess. Galls produced on a wide range of plants 

 by the deposition in them of the eggs of certain insects, notably the cynipid 

 wasps, are highly specific in form and often complex in structure. Tumors and 

 fasciations of various kinds are not infrequent. All these provide opportunity 

 for a wide field of research on form-determining factors, a whole science in 

 itself and destined to be of great importance in morphogenesis. Nothing quite 

 like them is known in the animal kingdom. 



Tissue culture, of course, is the almost complete collapse of organization. In 

 plants, however, it is rarely possible to produce the real tissue culture that 

 has been developed with animal material, where a particular type of cell 

 multiplies indefinitely. A certain amount of differentiation is present in most 

 plant cultures, and such cultures have a strong tendency to form growing 

 points and produce organized bodies again. The degree of this tendency is 

 affected by various external factors, notably some of the growth substances, 

 a fact that provides a direct means of studying the phenomenon of organiza- 

 tion itself. 



Roots, leaves, fruits, and even whole plants may now be cultured in sterile 

 media, and this makes possible a study of development under a completely 

 controlled environment. This has already been of much value for the solution 

 of physiological problems, but its great promise for studies in morphogenesis 

 has yet hardly begun to be utilized. 



