726 



ECOLOGY 



crowded cultures, and probably the death of the lower branches of trees 

 in the forest, is due to insufficient light; if a forest tree is left standing 

 when its neighbors are cut, the trunk often develops adventitious leafy 

 shoots in great abundance, probably because it is more exposed to light. 

 In various trees {e.g. willows and poplars) there occurs "self -pruning," 

 or branch fall, that is not readily referable to definite factors. 



Light as a factor in elongation. — The causes of elongation in crowded 

 cultures are not certainly established, though they are clearly external. 

 Sometimes the differences to be accounted for are very great, as in the 

 palmetto, which in dry open habitats often is stemless above ground, 

 while in moist woods, plants of equal age have long, slender trunks 

 several meters in height. When the prickly pear (Opuntia) grows in 

 the light, the stems become much flattened (fig. 1042), while in dark- 

 ness they become slender, elongated cylinders, somewhat comparable 

 to their "juvenile" stems (figs. 1040, 1041). The stems of Genista 

 develop flattened wings in the light but not in darkness. In germi- 



1045 



Figs. 1043-1045. — Stem and leaf variation in Sempervivum assimile: 1043, a stem- 

 less rosette, as seen in nature, having imbricated leaves in many ranks ; 1044, an individual 

 that has been grown in a moist chamber ; note the conspicuous erect stem with its terminal 

 rosette ; 1045, an individual that has been grown in a moist dark chamber ; note the small 

 and scattered leaves. — After Brenner. 



nating potato tubers, slender elongated stems issue from buds located 

 below the soil level, while short and very stout shoots may issue from 

 buds above ground (fig. 1046). The stipes of Mucor and Coprinus 

 elongate in the darkness and are relatively short and stout in the light. 

 These and similar facts have led to the view that light retards, and 

 that darkness favors elongation. 



