676 SECONDARY GROWTH IN THICKNESS 



summer in which the crown of a tree suffered serious injury through 

 the ravages of insects, will he represented by a narrow annual ring in 

 the stem, whereas seasons favourable to the development of foliage 

 will be marked by the addition of wide annual rings. In this way the 

 fluctuations in the width of the successive annual rings in the trunk 

 form a permanent record of the principal episodes in the history of the 

 tree. 



In oblique or horizontal branches and roots, the annual rings 

 are usually all asymmetrically developed. Thus, the upper half 

 of each ring is thicker in Acer, Alnus, Carpinus, Corylus, Gytisus 

 Laburnum, Fagus, Tilia, Robinia, and many other deciduous trees, 

 while the lower half is favoured among Conifers and also in Buxus 

 sempervirens and Viscum album?' C. Schimper, who was the first to 

 study excentric thickening in detail, applied the terms epinastic to the 

 former, and hyponastic to the latter of these two modes of growth. 

 Wiesner speaks of epitrophy and hypotrophy in the same connection. 



The origin of such inequalities in the growth of the upper and 

 lower sides of branches or roots is still imperfectly understood. 

 Several physiologists associate this phenomenon with the influence of 

 gravity ; others attribute it to the unequal incidence of temperature, 

 light and moisture upon the two sides of the organ. It is probable 

 that the distribution of mechanical strains within an organ plays a 

 leading part in producing asymmetrical thickening ; thus the upper 

 side of a horizontal or oblique branch is mainly subjected to tension 

 and the lower side to compression. With regard to horizontally ex- 

 tended subterranean roots, Kny states that these organs only exhibit 

 asymmetrical growth in thickness, when they are laid bare, and con- 

 sequently exposed to the atmosphere ; the woody cylinder is then 

 subject to the same influences as it is in the horizontal branches of 

 the same plant, and accordingly assumes a somewhat similar ana- 

 tomical structure. Wiesner, finally, has shown that asymmetrical 

 growth in thickness may also be brought about by internal influences. 

 If, for example, a lateral branch of Abies is forced to develop in an 

 upright position, the abaxial side of the woody cylinder still develops 

 more strongly than the opposite face (exotrophy). 



An epinastic or hyponastic tendency on the part of a twig or branch 

 may result in very marked vertical expansion of the organ, a fact already 

 noted by the author in his Botanische Tropenreise, in connection with 

 a " candelabrum tree " belonging to the genus Garuga (Burseraceae). 

 Here the basal curved region of each main branch is strongly flattened 

 in the vertical plane. In such a case as this the mechanical advantage 

 of the asymmetry is fairly obvious. As a result of the excentric 

 development of the annual rings, the cross-section of the branch 



