Grossenbacher Radial Growth in Trees. 49 



lar periodicity even in regions of the tropics having what ap- 

 pears to be a practically non-periodic climate. 



In central Uruguay" where the temperature never goes much 

 below freezing and where late summer is a dry season, some trees 

 have distinct yearly wood-rings, while in others more than one 

 ring is produced in a year. R&binia Pseudacacia and Melia 

 azedarach have fairly evident annual zones, but they also have 

 imperfect secondary zones due to a concentric arrangement of 

 large vessels. In Acacia the yearly zonation is less distinct but 

 the last wood is usually made up of cells with a reduced radial 

 diameter. 



The measurements by Hall 100 show that the trunks of trees in 

 Uruguay usually increase in circumference during nearly ten 

 months of the year, and that in some cases they even increased 

 during the months of May and June (winter). He found, how- 

 ever, that the circumference of most trees decreased more or less 

 during winter, the deciduous trees more noticeably than the ever- 

 greens. Ursprung 101 found that a number of the evergreen trees 

 and shrubs of a tropical locality without any appreciable peri- 

 odicity of climate showed a zonation in cross sections of the stems 

 without the presence of any evident histological difference in the 

 wood of the different parts of zones. Some of these species are 

 said to become deciduous in localities having a periodicity in the 

 water supply with the result that the zonation of their wood be- 

 comes more marked. Holtermann 102 also studied the relation of 

 climate to radial growth in the tropics and came to the conclu- 

 sion that the formation of growth rings in the wood is intimately 

 connected with the occurrence of periods of markedly different 

 transpiration rates, and that the larger vessels are developed to 

 meet the demands of increased transpiration. He holds that 

 tropical trees growing in a saturated atmosphere most of the 

 time have no indication of zonation in the wood even though they 



99 Christison, D. On the difficulty of ascertaining the age of certain 

 species of trees in Uruguay, from the number of rings. Trans. Bot. Soc. 

 Edinburgh. 18:447-55. 1891. 



100 Hall, C. E. Notes on the measurements, made monthly at San 

 Jorge, Uruguay, from January 12, 1885, to January 12, 1890. Trans. 

 Bot. Soc. Edinburgh. 18:456-68. 1891. 



101 1. c. 



102 Holtermann, C. Der Einfluss des Klimas auf den Bau der Pflan- 

 zengewache. Anatomisch Physiologische Untersuchungen in den 

 Tropen. pp. 249. 1907. Leipzig. 



4 S. A. 



