376 BOTANICAL GAZETTE [NOVEMBER 
dominance; its increased development in high latitudes and alti- 
tudes is not so easily explained. VoGTHERR (13) correlates the 
layering tendency with a moist habitat, made possible by the low 
evaporation rate in northern and mountain forests, and it is doubt- 
less true that moisture and absence of light are the factors that 
stimulate the buried portions of the branch to root production. 
But cases were noted (see example 4 above, and also SCHROTER 
11), in which, although the end of the branch became erect and 
treelike, no portion was buried, and therefore no roots were formed. 
In other cases trees with layered branches were found growing in 
xerophytic situations upon the exposed rocky shores of Lake 
Superior. Timberline conditions, too, more often than not, are 
xerophytic in the extreme. The connection with a moist habitat 
thus seems not to be an essential one. In explanation of the 
striking cases of layering reported from the tree line in various 
regions (circles and double circles of daughter trees surrounding 
the parent), it may be noted that in such localities the forest is 
open, and the trees therefore, on account of abundance of light, - 
are clothed with living branches to the ground. Moreover, they 
are as a rule short, bushy, and branchy, and the low crown tends 
to spread horizontally rather than to increase in height. Just 
such conditions as are found here (numerous healthy branches 
close to the ground) are those which would apparently most favor 
the appearance of the layering habit. In the endeavor to solve 
the problem, however, the meagerness of the data should be borne 
in mind. It may be that more extended and careful observation 
would prove that the habit is as common at low latitudes and alti- 
tudes as at high. Possibly the greater number of reports from arctic 
and alpine regions is due to the fact that the phenomenon is most 
easily observed there, or that individual cases of more striking 
appearance have been found. On Isle Royale, though the habit 
was exceedingly common, no such remarkable examples were dis- 
covered as those reported by Loupon and KimLMAN. 
IV. Ecological importance of layering 
The habit of layering, in regions where it occurs, must be 
included as an important factor in any investigation of forest 
