75° 



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 

 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 



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 (b') ; 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 {p)\ this alga is a coenocyte, being from the 

 outset without internal cell walls; considerably magnified. 

 — After Noll. 



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 



