126 



The Phenomena of Morphogenesis 



Fig. 6-5. "Inversion of polarity." Etiolated pea seed- 

 ling with epicotyl decapitated, inverted, and placed in 

 water. Roots now grow out from the epicotyl and a 

 shoot from a cotyledonary bud. (After Castan.) 



( 1915 ) found that apparent reversal of polar behavior in Coleus was only 

 temporary. 



Reversal seems to be easier to accomplish in seedlings than in older 

 plants. Castan ( 1940 ) cut off the epicotyl and the primary root from 

 etiolated pea seedlings and inverted them. Roots then grew out from the 

 originally apical end and shoots from the basal one (Fig. 6-5). Rath- 

 felder ( 1955 ) confirmed this observation and believes that it is a real 

 reversal. 



Reversal of polarity is much easier to accomplish in the lower plants 

 (p. 138). 



POLARITY AS EXPRESSED IN INTERNAL STRUCTURE 



Polar phenomena are manifest not only in external form but in in- 

 ternal structure. This is evident in many ways. 



Embryonic Development. In vascular plants the first manifestation of 

 polar behavior is in the division of the fertilized egg. This in most cases 

 seems to be related to the polar character of the gametophyte. Wetter 

 ( 1952), confirming earlier work of others, finds that in ferns the planes of 

 division in the young embryo are related to the axis of the prothallium 

 and that the segment that will form the first leaf is always directed 

 toward the growing point (notch), a fact also evident later in the orien- 

 tation of the young leaf itself. This relationship persists regardless of 

 the direction of the incident light. In Isoetes, the first division of the 

 fertilized egg is at right angles to the axis of the archegonium, and 

 early embryo development is not affected by external factors (La Motte, 

 1937; Fig. 6-6). 



