632 Harper. — Defoliation : its Effects upon the 
seriously interfered with the growth- increment upon which the forester 
naturally bases his expectation of financial returns. 
2. Effects on the Structure of the Wood. 
The effects of defoliation upon the amount of growth and the con- 
sequent breadth of the annual growth-ring having now been described, there 
remain to be considered some modifications which have been brought 
about in the structure of the timber. Essentially these changes consist in 
a decrease in the ratio which the thick-walled ‘ mechanical * elements bear 
to the porous water-conducting tissue, and since in a tree like the larch the 
spring wood consists entirely of thin-walled tracheides, while the thick- 
walled cells are massed together towards the outside of the ring as the zone 
of autumn wood, it is in this outer region that the histological effects of 
defoliation become apparent. There are two distinct ways in which the 
autumn wood may be affected. Firstly, its breadth relatively to the 
breadth of the whole ring may decrease, but without any accompanying 
diminution in the characteristic thickening of the cell-walls. Secondly, this 
normal thickening may be poorly developed or even wholly lacking for part, 
especially the outer part, or even the whole of the breadth of the autumn 
zone, so that it is only from the smaller size of its outer cells and their 
flattened shape at the very limit that the ring can be demarked from the one 
that succeeds it (Fig. 3), and in extreme cases the boundary is no longer 
to be traced with any accuracy (Fig. 9, between A and b). 
Now the autumn wood of the first year to show evidence of the 
attack of the Sawfly is never normal ; it is affected in the second of the 
two ways just mentioned, being deficient in the thickening of its cell-walls. 
In subsequent years, however, in spite of continued repetition of the defolia- 
tions, it may consist once more of normally thickened tracheides, in narrow 
zones commensurate with the decreased breadth of the whole ring (cp. 
Figs. 1, 2, 4, 6, 9, and Tables III, V, VII). But such a return to normally 
thickened autumn wood never occurred in such trees as ultimately suc- 
cumbed before they were felled for the purposes of this investigation. 
Fig. 8 shows a typical example of the appearance of the last-formed ring 
of one of these, and may be contrasted with Fig. 4, taken from the tree C, 
which was still vigorous at the time. On the other hand, examples D and 
F formed in the last two years before felling even an increasing percentage 
of quite normally thickened autumn wood (Fig. 1). They were said to 
have been only partially defoliated in those two years, but the betterment 
seen in the development of their autumn wood is probably referable rather 
to the fact already mentioned, that the plantations from which they came 
were thinned at that time ; for even tree E, from the same plantations, made 
at the top an increased proportion of autumn wood, though not properly 
thickened, in the very year in which it succumbed (Table VI). 
