634 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



development. The dorsiventrality and symmetry or asymmetry of the 

 leaf bud and of parts of flowers develop in a particular relation to the 

 axiate pattern of the part on which they arise. The whole pattern of 

 the multiaxiate plant represents a series of such relations. 



In certain coelenterates somewhat similar relations appear. Various 

 workers with hydra have observed that the pattern of tentacle appear- 

 ance on the bud of Pelmatohydra oligactis is not primarily radial but is re- 

 lated to the longitudinal axis of the parent hydra in such a way that 



Fig. 203. — Order of tentacle appearance in bud of Pelmatohydra oligactis. Upper row, 

 side views of stages up to four tentacles; second row, apical views of stages from two to four 

 tentacles; lower row, lateral and apical views of later stages (from Rulon and Child, 1937a). 



the bud is temporarily "dorsiventral."^^ The first two tentacles appear 

 simultaneously on opposite sides of the bud in a plane at right angles to 

 the polar axis of the parent; the third tentacle is midway between the 

 first two on the side toward the apical end of the parent; the fourth op- 

 posite the third; and the fifth and sixth between the first two and the 

 third, either simultaneously or one earlier than the other. Stages of this 

 development are shown in Figure 203. Gradually the tentacles become 

 equal in length and equidistant on the hypostome, and the dorsiventral 

 pattern becomes radial. Although the bud represents a certain degree of 

 physiological isolation and a new longitudinal axis at right angles to the 

 old, the parental pattern still plays a part in determining the order of 



'5 Rulon and Child, 1937, and their citations. 



