352 LECTURE XV. 



and the cells formed are relatively large : but as the summer 

 goes on the tension increases, owing to the continued forma- 

 tion of wood and bast by the cambium, and the size of the 

 newly formed cells diminishes. De Vries has proved the 

 correctness of this explanation by artificially increasing the 

 tension in branches in the spring by means of a tightly 

 wrapped ligature of twine, and observing that the cells formed 

 under the ligature were smaller than those formed beyond the 

 limits of the ligature. Knight's experiments supply evidence 

 of the same kind. He fixed young apple-trees so that the 

 lower part of the stem could not be moved by the wind, 

 leaving the upper part free to move. He found that the 

 upper free part increased in diameter much more rapidly than 

 the lower fixed part. The explanation of this difference is 

 that, owing to the swaying caused by the wind, the cortical 

 tissues of the upper free portion were stretched, and therefore 

 the pressure exercised by them upon the growing cells in the 

 cambium-region was smaller than that in the lower fixed 

 portion of the stem. 



The existence of this considerable transverse tension and 

 its effect upon the development of the tissues has long been 

 recognised in horticulture. It is a common practice to split 

 the cortex of young trees in the summer with a view to pro- 

 moting the formation of woody tissue. The consequence of 

 this is that more water can be conveyed upward to the 

 growing parts, and an increased formation of buds and leaves 

 is brought about. 



A further illustration of the effect of this transverse ten- 

 sion upon the development of the tissues is afforded by 

 excentric stems and roots, in which, namely, the annual rings 

 of wood and the cortex are thicker at some parts than at 

 others. This is due to local differences of tension ; where 

 the tension is the smallest the growth in thickness is the 

 most rapid, and conversely. The differences of tension are 

 due to a loosening of the cortex at certain parts which may 

 be brought about by various causes. Thus Knight observed 

 in the case of young apple-trees which were so secured that 

 they could be swayed by the wind only in a plane lying 



