128 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
process will still be going on, so that by the fiftieth year or 
so, tall, clean, and moderately stout stems will form the remain- 
ing crop; the weak and suppressed trees having been removed 
at short intervals since the first thinning. Here, then, we have 
one condition assured—viz., clean growth—and we have now to 
consider its quality. 
As already mentioned, quality cannot be considered by itself in 
cultivated crops, but in relation to its cost of production and the 
increased selling value which accompanies it. Quality in timber 
consists in its size and maturity, as well as its freedom from knots. 
A crop of clean timber at fifty years of age may therefore be lack- 
ing in the required size and maturity which is necessary in order to 
command the most profitable price, and growth must be encouraged 
to proceed until these several conditions are fulfilled. The opera- 
tion usually employed for encouraging the growth of individual 
trees in a thick wood is thinning. In this country, so much 
importance has been attached to this work, that we find it carried 
out in plantations at all stages of their growth. Judicious thinning 
is always advantageous, but much plantation timber is spoilt during 
the early stages of growth by mistaken ideas on this subject. 
Unless the killing-off or suppression of side branches takes place 
before these have attained a diameter at the base of two or three 
inches, rough timber is produced, and no amount of skill in the 
after-treatment of the plantation can remedy this defect. We have 
seen that German foresters scarcely thin at all during the early 
stages, and the result is clean poles, with a certain proportion of 
fairly stout trees. It is the existence of a properly distributed pro- 
portion of the latter which furnishes a reliable criterion by which 
to judge of the value of the system and its correctness. On good 
soils and situations the growth of trees is fairly rapid, and the 
differences in individual vigour become more marked year by year, 
and weak individuals are quickly suppressed. In such cases the 
trees “thin themselves,” as some express it, fairly satisfactorily, and 
but little assistance is required from the forester. On poor soils 
and bad situations the struggle for supremacy is slow and pro- 
longed, and the trees are less able to bear the weakening tendency 
which accompanies crowding. The result may be, therefore, that 
too great a proportion of the trees are too weak and spindly to 
develop into good stems, and a crop of low value may be produced. 
While, in the former case, the forester may leave the thinning pretty 
much in Nature’s hands, and simply remove those trees which have 
