232 The Phenomena of Morphogenesis 



remain alive and are potentially meristematic. These primordia, often very 

 numerous, remain dormant under ordinary conditions. 



This fact raises the question of what it is, if these primordia are 

 capable of growing and forming new organs, that prevents them from 

 doing so. The concept of the organism as a balance between stimulatory 

 and inhibitory factors suggests itself here, but the problem remains as to 

 what localizes and correlates the activitv of these factors so that a specific 

 organic system is established, maintained, and restored, and what stops 

 the regenerative process when this has been accomplished. 



It should be remembered that, in spite of modifications acquired during 

 the process of differentiation, all the cells are probably identical geneti- 

 cally, save for occasional polyploidy. The potentially meristematic cells 

 thus serve, so to speak, as a "germ plasm" or genetic reserve which can 

 direct the processes of regeneration and further development. Each cell, 

 at least in theory, is capable of producing an entire individual. 



In regeneration, mature or nearly mature cells may sometimes become 

 embryonic again and then undergo changes that restore a disturbed tissue 

 pattern. How this is accomplished is of much interest for morphogenesis. 

 In it the first visible step is usually a marked increase in the amount of 

 cytoplasm and in the size of the nucleus and an acceleration of metabolic 

 activity. The wall also tends to become thinner. This process, commonly 

 termed dedifferentiation, has been described and the literature reviewed 

 by Buvat ( 1944, 1945, 1950 ) . Dedifferentiated cells assume the character 

 of meristematic ones and can divide and grow. The tissue thus formed 

 may then differentiate again in conformity with its new function or 

 position in the regenerated structure. 



REGENERATION IN THE LOWER PLANTS 



Among the thallophytes and bryophytes, with their simpler bodies and 

 lower level of organization, regeneration is relatively common. It will be 

 discussed briefly in these groups before taking up the more complex 

 aspects of it that vascular plants display. 



In most of the lower plants single cells or groups of cells have the 

 ability to develop readily into a whole plant, and in many instances they 

 do so regularly as a means of vegetative reproduction. Extreme instances 

 of this are the conversion of vegetative cells into specialized nonsexual 

 reproductive cells such as zoospores and others. 



Even when not thus differentiated for reproduction, the cells of many 

 algae are readily separable from the loosely organized thallus and grow 

 into new plants, as in Callithamnion (Weide, 1938) and Cladophora 

 (Schoser, 1956). In the last genus the cells may be separated from one 



