100 THE SYLVICULTURAI. SYSTEMS. 



At all times the youngest age gradation toinis iniit of the 

 underwood, until the latter is cut over. 



Every standard removed at the entl of the rotation must 

 be replaced hy younger trees. If all seedlings reached the 

 end of the rotation, only one need he planted for every full- 

 sized standard wliicli has been removed, hut as numerous 

 seedlings have, owing to various causes, to he taken out before 

 the end of the rotation, it is necessary to plant several seed- 

 hngs for every mature standard removed, so that, as a matter 

 of fact, the numbers of the standards in tlie several age 

 gradations always form a falling series. The actual propor- 

 tion of trees in each age gradation depends on circumstances. 

 In the above mentioned example the proportion would be 

 somewhat as follows : — 



Number of age gradation . . .1. II. HI. I^ • V. 

 Proportion of standards . . . "iO I'i -5 '2 1. 



In other words, for every desired standard 100 years old or 

 more, it is necessary to have 20 standards 20 years old, which 

 will be reduced to 12 at 40 years, to 8 at ()0 years, to 2 at 

 80 years, and to 1 at 100 years old. Though 38 standards 

 are present on a certain area, only five reach the end of 

 the rotation, the others being cut from time to time, thrown 

 by wind or otherwise injured. 



The actual number of standards per unit of area is governed 

 by considerations for the underwood. If the standards ai-e 

 very numerous and give too nuich shade, the underwood will 

 no longer thrive. In practice a great variety of modifications 

 have been introduced, according to whether the overwood or 

 underwood is more favoured. 



The normal form is that in which over- and underwood 

 receive equal attention. Here the efiect of the shade thrown 

 by the overwood must always be carefully considered; 

 generally, the shade should be evenly distributed over the 

 area. It is always desirable that the bulk of the overwood 



