THE STRUCTURE OF THE ROOT 21 



through the vessels. If the leaves exert but little suction, the 

 root pressure forces the water more strongly up the stem. A 

 given amount of pressure will act more vigorously in this way 

 the smaller the friction is within the vessels. The resistance 

 due to such friction diminishes with the increasing diameter 

 of the vessels. It is for this reason that climbing plants 

 (Vitis, Tecoma, &c.), which have to carry large quantities of 

 water to considerable distances, are provided with very wide 

 vessels. 



In the case of narrow vessels, however, another force comes 

 into play to a considerable extent, namely, capillary attraction. 

 The attraction of water by the wall of very narrow vessels 

 causes the liquid to creep up in the thin tubes, where it is 

 often retained in considerable columns, even though air should 

 enter at the bottom. In such cases we often find small columns 

 of water interrupted by columns of air, the whole forming an 

 interrupted column of water and air, termed Jamin's chain. 



This chain-like arrangement, which is formed very frequently 

 when rapid transpiration takes place above ground without 

 sufficient supply of water from the root system, makes it 

 possible for small quantities of water to be distributed over 

 the whole length of the vascular system, so that the various 

 organs will all be supplied, should they draw still further on 

 the limited supply. In this way they can be preserved from 

 drying up altogether. This would not be possible if the water 

 were to sink to the bottom of the vessels. Besides this, the 

 small quantities of water in the tubes can move about more 

 easily when forming a Jamin chain, and can therefore more 

 readily supply the thirsting cells at various levels of the stem. 

 For when the outer temperature rises, and consequently the 

 temperature within the plant gradually increases, the air ex- 

 pands within the vessels and raises the intervening drops of 

 water. 



Thus, while the sucking and forcing actions of the con- 

 ducting tissues are specially efficient in the rapid conduction 

 of large quantities of water, the capillarity of the vessels and 

 the formation of Jamin's chains are of especial benefit in 

 the case of a reduced water supply. But besides this, a con- 

 tinuous interchange of water takes place within the walls of 



