Morphogenesis 583 



Normal morphogenesis occurs when the correct balance between spe- 

 cific stimulators and susceptible structures is present. Occasionally, cer- 

 tain tissues will abnormally assume growths known as neoplasms (ne' o- 

 plazm) (Gr. neos, new; plasma, formation). If such a growth is harm- 

 ful or malignant, we commonly refer to it as a cancer or carcinoma (kar'- 

 sinomah) (Gr. karsinos, crab, cancer). We commonly use the word 

 tumor (tu'mor) (L. tumere, to swell) if the growth in general is harm- 

 less or nonpathologic. Sometimes a neoplasm may grow only at its 

 original site, but occasionally certain of the abnormal cells may circulate 

 to other areas by metastasis (me -tas' ta sis) (Gr. met a, change; stasis, 

 place) and initiate abnormal growths in new places. 



Plants also possess morphogenesis whereby tissues and organs are dif- 

 ferentiated and developed. Quite different tissues and organs of plants 

 must originate differentially from what appears to be similar cells and 

 tissues. How can a plant develop such different structures from appar- 

 ently the same cells and tissues? As in animals, so plant morphogenesis 

 is influenced by the hereditary potentialities of the various species as well 

 as by the presence of specific substances. Not only are specific genes 

 involved, but plant hormones known as auxins (ok' sin) (Gr. auxein, to 

 increase) initiate and regulate many phases of plant growth and develop- 

 ment. Auxins are organic acids which are synthesized by the living 

 protoplasm in certain parts of plants. These auxins are specific, at least 

 to a certain degree, and may affect organs in which they are formed or 

 other tissues and organs to which they may be transferred. The latter 

 seems to be primarily polar; that is, they are transferred largely in one 

 direction. A more detailed consideration of auxins and their actions is 

 given elsewhere. 



QUESTIONS AND TOPICS 



1. Learn the correct pronunciation, spelling, and derivation of each new term 

 used in this chapter. Include as many typical examples of each as possible. 



2. Contrast homology and analogy and give as many examples of each as pos- 

 sible. 



3. Explain why it might be desirable or undesirable for living organisms to 

 possess autotomy generally and extensively. 



4. What benefits have you derived from a study of homology, analogy, autot- 

 omy, regeneration, and morphogenesis ? 



5. Explain why differentiation and growth are not necessarily synonymous. 



6. Explain how each of the various factors may be influential in the process of 

 differentiation in morphogenesis. 



7. What is the relationship between genes and environmental chemical sub- 

 stances in the determination of specific traits ? 



