422 PATTERNS AND PROBLEMS OF DEVELOPMENT 



other physical or chemical factors at the two ends of Tubularia or Cory- 

 niorpha pieces would have effects very similar to those of electric current ; 

 but, except for temperature difference, reported to be effective in Cory- 

 morpha by Gilchrist and Schmidt (1932) and in Tubularia by J. A. Miller 

 (1939), and for difference in H-ion concentration, also stated to be effec- 

 tive,^'^ experimental evidence seems to be lacking. 



In many plants gravity is a factor in localizing regions from which 

 rhizoids or roots develop and in determining what bud primordia shall 

 develop, but apparently it is not an essential factor in determining the 

 root or bud gradient. Its effect in these cases appears to be determination 

 in certain regions of conditions favorable for initiation of rhizoid or root 

 formation. Such determination may, of course, play a part in establishing 

 a general polarity of the whole plant or piece, but the polarity of each 

 rhizoid or root apparently results from the local activation and initiation 

 by it of a gradient system. In buds activated by gravity the gradient 

 pattern is already present, and gravity appears to be merely an activator. 

 Gravity is supposed to bring about differential distribution of substances 

 of different specific gravity and may determine gradients in concentration 

 of such substances, but these gradients are not essential to the axiate 

 patterns of particular rhizoids, roots, or buds. 



Prolonged low-speed centrifuging of isolated cells of the alga Griffithsia 

 tends to localize development of new apical cells and resulting thallus 

 axes centrifugally, where heavier substances are concentrated; but the 

 concentration of substance is regarded as stimulating or activating rather 

 than as directly determining the new polarity (Schechter, 1935). 



According to Loeb (1891), pieces of the hydroid A ntennularia antennina, 

 suspended in various positions, develop stolons from parts toward the 

 earth, hydranth-stem axes from parts extending in the opposite direction. 

 Stolons develop even from the apical ends of lateral branches or of the 

 main axis when these point downward. Further experiments by Morgan 

 (1901) and Stevens (1902, 1910) do not entirely confirm Loeb's results. 

 These authors find that stolons often develop from both ends of sus- 

 pended pieces and that pieces in various positions on a slowly revolving 

 vertical wheel in water usually develop hydranth-stem axes. As in the 

 case of Plumularia, this latter result is probably due to better oxygen 

 supply and removal of CO2 with the constant change of position on the 



'^ According to Komori (1933), pieces of Tuhularia reconstitute stolons or nothing from the 

 distal ends at pH 6 and hydranths from proximal ends at pH 8.45. This difference in H-ion 

 concentration is not great and raises the question whether hydrogen ion or CO^ is the effective 

 factor in this case. 



