ANNUAL RINGS 675 



the apical ends being placed in water. The explanation offered by 

 Fischer is that the ascending water carries glucose in solution with it ; 

 but since this substance is not utilised at the upper end of the twig, it 

 becomes converted into transitory starch. 



Experiments performed by Hartig and Fischer indicate that carbo- 

 hydrates travel downwards, but not upwards, in the secondary phloem ; 

 Fischer accordingly concludes, that all the synthetic products deposited 

 both in the secondary phloem and in the woody cylinder are carried 

 upwards by the transpiration current in spring-time, always excepting, 

 of course, the portion which is used up locally in connection with 

 respiration and growth. The possibility that the xylem-parenchyma 

 participates in the longitudinal transportation of carbohydrates must, 

 however, never be lost sight of ; this matter, in fact, stands in urgent 

 need of further experimental investigation. 



3. Annual rings. 



In the temperate zones, the growth in thickness of trees and 

 shrubs is interrupted annually by the cold season. The addition 

 which is made to the woody cylinder during a single vegetative season 

 is often clearly distinguishable, owing to special anatomical arrange- 

 ments, from the immediately preceding and succeeding zones ; these 

 seasonal increments are usually known as the annual rings. The limits 

 of each annual ring are formed by the spring wood and the autumn 

 wood respectively. Similar annual rings may also develop in tropical 

 woody plants, especially where vegetative activity is interrupted by a 

 regularly recurring dry season. 361 



In erect trunks and branches, the annual rings are in general 

 equally broad all round ; deviations from this rule are almost always 

 referable to external influences. Mention has already been made in an 

 earlier chapter (p. 1 95) of an experiment performed by Knight, in which 

 the stem of a young Apple tree was fixed in such a way, that the wind 

 could only bend it from north to south or vice versa ; under these con- 

 ditions, growth in thickness was found to be greatest in the plane of bend- 

 ing. Later investigators, such as Nordlinger, Metzger, K. Hartig and 

 F. Schwarz, have also drawn attention to the connection between excen- 

 tric secondary thickening and the direction of the prevailing winds. 

 Ursprung has pointed out that excentric thickening of an erect stem, 

 or asymmetrical development of individual annual rings, will be 

 mechanically advantageous, if it accelerates the recovery of the stem 

 from any accidental distortion. 



Variations in the thickness of successive annual rings in one and 

 the same transverse section of a stem are principally due to differences 

 in the general conditions of nutrition in successive seasons. Thus, a 



