750 



ECOLOGY 



Attempts have been made to explain polarity by postulating the ex- 

 istence in plants of shoot-forming substances that migrate upwards and 

 of root-forming substances that migrate downwards. In a willow branch 

 new shoots develop toward the apex, where the shoot-forming sub- 

 stances are supposed to congregate in greatest abundance, and new roots 

 develop toward the base, where the root-forming .substances are thought 

 to be most abundant. Apparently favoring this theory is the fact that 



severed old leaves of 

 Begonia or of Achi- 

 menes regenerate shoots 

 that soon produce 

 flowers, whereas shoots 

 developing from young 

 leaves do not develop 

 flowers for some time, 

 as though old leaves 

 were much fuller of 

 flower-forming sub- 

 stances than are young 

 leaves. However, un- 

 til something is known 

 concerning these pos- 

 tulated substances, the 

 theory must be re- 

 garded as mystical if 

 not actually erroneous. 

 To some extent tend- 

 encies toward polarity 



1074 



FIGS. 1074, 1075. Reversal of polarity in an alga, 

 Bryopsis muscosa: 1074, an ordinary plant (somewhat 

 schematic) ; 1075, the apex of a plant that has been grown 

 in an inverted position; note that rhizoids (r) have de- 

 veloped from the apex of the shoot (a), and that shoot 



branches (6) nearer the original rhizoid pole have con- 

 tinued to develop as branches but have taken a new 

 direction (&') ; the dotted portions represent the original 

 part of the plant before inversion, while the undotted 

 portions represent portions growing after inversion; note 

 the intimate contact between the rhizoids and the soil 

 particles (/>) ; this alga is a coenocyte, being from the 

 outset without internal cell walls; considerably magnified. 

 After NOLL. 



may be counterbal- 

 anced by external fac- 

 tors; for example, if a 

 willow shoot is laid 

 horizontally in the 

 water or on the soil, 

 shoots and roots often 



develop more or less 



equally along the whole length of the shoot, the former chiefly above and 

 the latter below; gravity, light, and water probably enter here as factors 

 of importance. In Zamia, shoots may appear at both ends of a stem 



