236 



BOTANY 



PART I 



This shows that the cavities of the vessels are essential for water 

 conduction. In the living plant, however, the vessels and tracheides 

 always contain air in addition to water, at least when transpiration 

 is active. 



In water plants and succulents, in which little or no transpiration takes place, 

 the xylem is correspondingly feebly developed. On the other hand, the transpiring 



leaf-blades have an extraordinarily rich 



h supply of vascular bundles ; these anasto- 



mose freely, so that any particular point 

 is sure to obtain sufficient water. The 

 illustration (Fig. 126) gives some idea of 

 this irrigation system of a leaf-blade, but, 

 since the finest bundles are only visible 

 with the help of the microscope and are 

 not represented, the system is even more 

 complex. The conducting tracts in the 

 stem leading to the leaves form, especi- 

 ally in trees which grow in thickness, a 

 wonderfully effective conducting system. 

 All the wood of a thickened stem does 

 not seive this purpose ; water conduction 

 is limited to the more recently developed 

 annual rings. When a heart -wood (p. 

 158) is formed this takes absolutely no 

 part in the process. 



There is still uncertainty as to 

 the forces which give rise to the 

 transpiration stream. It is natural 

 to think of a pressure acting from 

 below, or a suction from above, 

 and to regard the former as due to 



FIG. 244.-HALES' experiment to show the ascent roO t-presSUre, the latter to the prO- 



of the sap in the wood. Although the cortex r . , 



has been entirely removed at Z, and the wood C6SS Ot transpiration. I here are, 



alone left, the leaves of the branch b remain however, a number of reasons 



as fresh as those on the uninjured branch c ; againgt ascr ibing the movement of 



x, vessel containing water. Facsimile of the o 



illustration in HALES' Vegetable Statics, 1727. the Water to rOOt-preSSUre, and 



whether the suction force exerted 



by transpiration is sufficient to continuously raise water to the summit 

 of a high tree appears doubtful. No generally accepted solution of 

 the much -discussed problem of the ascent of water has yet been 

 attained. 



The following points have to be considered as regards the root-pressure. In 

 many plants the root-pressure actually observed is very slight or absent. Even 

 in plants with a powerful root-pressure the -amount of water thus supplied in a 

 given time is considerably less than that lost in transpiration. With somewhat 

 more active transpiration, therefore, the root- pressure is not manifested in the 

 way described above. When an actively transpiring plant is cut across above the 



