580 Animal Biology 



hydra too small to regenerate themselves may fuse together and the 

 mass then form a new hydra. Parts of one hydra may easily be grafted 

 upon another. 



If a planarian flatworm (Dugesia) is cut into two pieces, the anterior 

 part will regenerate a posterior portion, while the posterior part will 

 regenerate a new head. A middle piece may regenerate both head and 

 posterior end. The head may regenerate another head, in rare instances. 



A posterior end of an earthworm may regenerate an anterior end 

 (Fig. 28). An anterior piece regenerates a posterior part. A posterior 

 end under certain conditions may regenerate another posterior end 

 which results in the eventual death of the individual. Pieces from sev- 

 eral worms may be united (grafted) to form a longer worm. 



In higher and more complex organisms the process of regeneration is 

 more or less limited if not lacking entirely. In general, the less spe- 

 cialized tissues and structures have greater powers along this line than 

 the more specialized structures. In man, such tissues as blood, bone, 

 skin, etc., are replaced, while other tissues and organs are not. 



Certain tissues which are injured or lost may be regenerated by the 

 active tissues of a plant. The process is dependent upon (1) the quan- 

 tity and quality of the auxins (plant hormones) present which initiate 

 and control the growth, (2) an adequate supply of water, (3) a suffi- 

 cient amount of energy and building materials supplied by such foods 

 as carbohydrates and proteins, (4) the particular type and age of the 

 tissue involved, etc. In general, the natural, inherent process of regen- 

 eration in plants starts with the forming of a protective layer over the 

 injury and called a callus (ka'lus) (L. callium, hard skin). The latter 

 develops meristematic tissues from which the proper parts are regener- 

 ated. Roots may be regenerated on such stems as coleus, geraniums, 

 willows, roses, etc. Roots may be regenerated on the leaves of begonias, 

 African violets, etc. Stems may be formed from roots, and roots from 

 roots. Regeneration is frequently taken advantage of in the commercial 

 propagation of plants from cuttings. The latter phenomena are mate- 

 rially assisted through the use of certain plant hormones, which are con- 

 sidered in greater detail elsewhere in the text. 



MORPHOGENESIS (mor fo -jen' e sis) (Gr. morphe, form; genesis, 

 origin) 



This phenomenon includes the origin, differentiation, and develop- 

 ment of specific structures, organs, or parts of organisms. In normal 

 embryologic development of cells, tissues, organs, etc., of an organism, 



