48 Wisconsin Academy of Sciences , Arts, and Letters. 
water requirements of such habitats induce the formation of 
large vessels throughout the wood for the conduction of the 
water needed. This may be typically illustrated by a paper of 
von Lazniewski 93 * on alpine plans in which attention is called to 
the fact that the rings in mountain willows are much thinner and 
have a greater proportion of vessels per ring than those in trees 
of the same species grown in the valleys. Yet it was noted that 
the outer parts of the wood rings were usually only partially 
lignified, indicating that radial growth had been prematurely 
checked. The excessive number of vessels per ring of the alpine 
trees was interpreted as being due to the greater demands for 
water on the mountains, while the probable fact that the sum¬ 
mer-wood portion of the rings had perhaps been wholly elimi¬ 
nated by the environment was not even mentioned. Practically 
the same observations although on a larger scale were made by 
Rosenthal 96 in a later paper and the conclusion was drawn that 
the larger number of vessels per unit area of cross section in 
willows grown on the mountains is an adaptation to a higher 
transpiration rate. 
A number of hypotheses have been elaborated in an endeavor 
to explain “annual” rings, and more or less data has been col¬ 
lected by their supporters to substantiate them but with indiffer¬ 
ent success as judged by Krabbe 97 , who some years after publish¬ 
ing his last researches on the subject, maintained that ring 
formation cannot be satisfactorily explained with our present 
knowledge of the factors determining the size differentiating 
cells attain in different parts of the growing season, and of 
the ones regulating the thickness of cell walls in different parts 
of the rings. 
It was recently pointed out by Klebs 98 that periodicity in 
plant growth occurs in all regions of the world having a periodic 
climate, and that the dormant periods coincide with the cold pe¬ 
riods of temperate climates and with the dry periods of the 
tropics. He noted too, that some trees have partial and irregu- 
9G * Lazniewski, von, W. Beitrage zur Biologie der Alpenpflanzen. 
Flora, 82:224-67. 1896. 
98 Rosenthal, M. Ueber die Aushildung der Jahresringe an der Grenze 
des Baumwuchses in den Alpen. Inang. Dissertation. Berlin, pp. 24. 
1904. 
97 Krabbe, G. Einige Anmerkungen zu den neusten Erklarungsver- 
suchen der Jahringbildung. Ber. Deut. Bot. Ges. 5:222-32. 1887. 
98 1. c. 
