2 28 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 controlHng 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 DeVries (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 Krabbe (17, 18) beyond all contention in 1882, and since that 

 time a number of new theories have sprung into existence, each 

 with adherents. 



ROBT. Hartig (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 Hartig 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 Hartig'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, according to his theory, on the part of the plant 



