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. Robinia Pseudkcacia 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 w T ith 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. 
190 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. 
