174 



THE LIFE OF THE PLANT 



Despite, therefore, the many doubts cast upon 

 the subject, the part played by the vessels, as the 

 aqueducts of the plant, can no longer be disputed. 

 Together with the explanation of the part played by 

 the vessels, the significance of one more peculiarity 

 of their structure which long puzzled the most cele- 

 brated anatomists has also been explained. This is 

 the so-called bordered pit found in vessels and conduct- 

 ing fibres (tracheides) . They are easily observed in the 

 wood of our fir-trees, e.g. in a very thin section of almost 

 any match-stick. If we make a longitudinal section 

 in the plane of the axis and diameter of the trunk 



C D. . 



Fig. 50. 



(a radial section), we shall notice on the walls of the 

 vascular fibres numerous rings with double outlines (fig. 

 50, A, a and b). On closer observation we notice a 

 third less definite circle (c) lying between a and b. If 

 the longitudinal section does not lie in the plane of the 

 diameter of the trunk, but intersects it at a more or less 

 acute angle, the figure will change. Instead of the sur- 

 face view of the pit we shall see it at an angle, say of 

 45 (B), and ascertain that there generally are two 

 inner (small) rings. In order to understand the structure 

 of the pit more clearly, we must make a third section 



are shown under the microscope during vigorous evaporation of water 

 from the leaves. By comparing them we notice that drops of water have 

 decreased at c, while air bubbles have correspondingly increased. 



