°6 LIGHT IN RELATION TO TREE GROWTH. 
ter of fact, on a 10-year branch of birch grown in the open Wiesner 
(1907) Brad only 238, and on a branch grown in the shade only 182 
lateral branches; and instead of 9 successive orders of branches there 
were only 5. The reason for many of the buds failing to develop 
into shoots and the gradual dying of existing shoots must be sought 
chiefly in the minimum light intensities under which buds and 
foliage of a,given species can develop and exist. This is especially 
true of the buds, since, as has already been pointed out, the unfolding 
of buds requires higher lght intensities than the mere process of 
growth. It stands to reason that species capable of withstanding 
low light intensities will succeed in unfolding a proportionately 
larger number of buds every year and will preserve during their 
mature life a larger number of original ramifications than will light- 
needing species. 
Wiesner (1907) prepared the following lists of trees and shrubs, 
arranged according to the number of branch orders found in them: 
COMMON FOREST TREES. 
Maximum Maximum 
Species. ee Species. Dea 
orders. orders. 
Warix (archi. cee een aoe ee 3=45| ROUCRCUS! (Oaks) ssoeege see eee eee 6 
Gineko(cingiko) Wisse een eee 45) Robiniai (locust) seat cess eee 7 
Gleditsia (honey locust)......-.....-.-.- eal | amu Si (Clim) eee tess tae eee es eae a 
Ropulus(@poplan)s ae eee Gy ie dib-aanbks (PSN) se eek SES eB okodoaee 7 
Piceal(SPruce) assess eee cere ieee 5, ||-Carpinus (hornbeam) S25 so-aaeeee eee 8 
Pinus laricio (Austrian pine)..........- Lie Ind Brekeq bh Loteter0) OW) ee ee ee ar eee 8 
Betula (birch) *=2ss2e5-seese paclea nisi 54 PE ARUSIGVEW) Seams Saas aaeee eee eae 8 
SHRUBS. 
Cornus (dogwood) S252 osas-eeee eee ae 4 || Philadelphus (mock orange) ........-- 6 
Sambucusi¢elden)- se e= pene ee 6 || Ligustrum ( ) ected 7 
Viburmunt (aiburnum) 2 355.22 sess. 6 Sytinga ce: we Says Se ciee ae ee eee eee 7 
In these lists the species having the smallest number of branch 
orders must be classed as light- heen and those with the largest 
number of branch orders as eines: shade-enduring. The only ap- 
parent exception is spruce, which, by this method, would be grouped 
with the light-needing species. The small number of branch orders 
found in spruce may not, however, be due entirely to light, since many 
of the smaller branches of old spruce trees are bitten off by animals, 
especially squirrels. The interference by animals, insects, and other 
agencies with the successive branch formations in a tree constitutes 
a weak point in this method of determining lhght requirements of 
species; moreover, the method is, of course, altogether too complicated 
for practical use. 
NATURAL THINNING OF STAND. 
The rapidity with which stands thin themselves affords another 
indication of the hght requirements of trees. As soon as the crowns 
