Growth and Structure of the Wood of Larix. 633 
(a) Breadth of Zone of Autumn Wood. 
But the importance that might otherwise be attached to the breadth 
of the zone of autumn wood is perhaps increased by the consideration of the 
uppermost section of tree C. This section was taken seven feet below the 
tip of a well-grown tree with an exceptionally well-developed crown that 
extended, in fact, more than half-way towards the ground. Fig. 11 shows 
the east side of the section, where the outermost ring is composed almost 
entirely of autumn wood ; Fig. 12, from the west side of the same section, 
shows the same ring consisting of nothing but spring wood ; while on the 
south side of the section the ring disappears altogether (Text-fig. 1). Only 
four feet lower down (Text-fig. 2) it is the west side that shows the more 
striking development of autumn wood for this particular year, and the east 
side has about the normal proportion (about 50 per cent.), while the ring is 
very narrow and of unthickened tracheides on the north side and practically 
disappears again on the south. 
5 5 
Text-fig. 1. Text-fig. 2. 
Diagrammatic representations of the last year’s growth of the two uppermost sections of the 
same tree, the autumn wood being shaded (cp. Table VIII). 
The correct interpretation of these two sections requires some adequate 
understanding of the physiological causes of autumn wood formation. 
Clearly the difference between the east and west sides of the uppermost 
section could not be explained on Sachs’s 1 hypothesis that the characteristics 
of autumn wood tracheides are due to constriction of the cambial region by 
the encircling bark ; moreover, Krabbe 2 has proved that this supposed 
periodically recurrent compression of the expanding cells does not exist. 
Schwarz 3 refers the formation of thick-walled tracheides to the stimulus of 
longitudinal stress during bending by the wind. Wieler 4 and Hartig 5 
suppose it to be largely a question of nutrition, but whereas the former 
concludes that autumn wood is produced because conditions of growth are 
becoming less favourable, the latter considers that thick-walled cells are due 
to the extra nutriment that has been accumulating during the summer. 
Strasburger 6 and Haberlandt, 7 on the other hand, considering the function 
1 Sachs ( 1 ), p. 409. 2 Krabbe, p. 1125. 3 Schwarz, p. 365. 
4 Wieler, p. 129. 3 Hartig ( 1 ), pp. 34 and 103. 6 Strasburger, p. 949. 
7 Haberlandt, p. 371. 
