228 j BOTANICAL GAZETTE [MARCH 
known, each normal ring may be divided into spring wood and 
summer wood, or better early wood and late wood. The second 
of these two regions is distinguished from the first either by a 
diminution in the size of the vessels, as in the case of ring porous 
woods, or through a reduction in size and flattening of the elements 
formed in the outer part of the ring. The proportion of early and 
late wood in the ring affects strongly the physical properties of the 
wood, and as a result the early workers gave much time to its con- 
sideration. The factors controlling the amount as well as the 
time of late wood formation have been a subject of inquiry, and a 
hasty review of the literature on the subject, as well as a summary 
treatment of the results of this study from this viewpoint, are 
appropriate here. 
One of the first theories offered to account for the variation 
in ring was that of Kraus (19), SACHS (30), and DreVRIEs (36), 
who explained it through differences in bark pressure at different 
times during the growing season. The radial pressure was at a 
minimum in the spring, permitting a greater expansion of the new 
elements, while it gradually increased during the growing season, 
ending with a maximum. The pressure leading up to the last was 
responsible for late wood formation. This theory was disproved 
by KrasBe (17, 18) beyond all contention in 1882, and since that 
time a number of new theories have sprung into existence, each 
with adherents. 
Rost. Hartic (9) sought to explain the late wood formation in 
that the cambium was but poorly nourished in the spring. Late 
wood formation depended upon improvement in the nutritive 
conditions later in the season. According to Hartic the size of the 
lumina of tracheids is dependent on the amount of transpiration of. 
the foliage, while the thickening of cell walls is correlated with the 
increased amounts of food available at that time of the year. 
Diametrically opposed to Hartic’s theory is that of WIELER (37) 
and Russow (29), which was based on the assumption that the 
early wood owed its origin to better conditions of nourishment. 
STRASBURGER (32) accepts neither of these theories, but explains 
annual ring formation as a normal fixed process. The young wood 
is the response, Labi to his theory, on the part of the plant 
