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 95 * 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- 



S! * Lazniewski, von, W. Beitrage zur Biologie der Alpenpflanzen. 

 Flora, 82:224-67. 1896. 



" Rosenthal, M. Ueber die Aushildung der Jahresringe an der Grenze 

 des Baumwuchses in den Alpen. Inaug. Dissertation. Berlin, pp 24. 

 1904. 



87 Krabbe, G. Einige Anmerkungen zu den neusten Erklarungsver- 

 .suchen der Jahringbildung. Ber. Deut. Bot. Ges. 5:222-32. 1887. 



» 8 1. c. 



