Practical Plant Biology. 



conveyed into the wood of the conducting strand of the root. 

 There are special cells, passage cells, in the endodermis, opposite 

 the protoxylem filaments, which appear to be exceptionally per- 

 meable and facilitate this transit. The traction of the evaporating 

 surfaces in the leaves draws the water into the capillary tubes of 

 the wood and upwards through the wood of root, stem and leaf. 

 To understand how this takes place it is necessary to emphasise 

 a property of water which does not commonly come under 



FIG. 78. Ranunculus repens, conducting tract of root, transverse section, x 

 175. b, bast ; c, cortex ; e, endodermis ; px, protoxylem ; x, wood- 

 tracheae. 



notice. Water is capable of transmitting a pull and under 

 suitable conditions may, by its tenacity, or tensile strength, be 

 used like a rope or a wire to drag a weight. This tenacity 

 usually escapes our observation because the necessary conditions 

 to allow it to come into force are not realised. 



I have here a glass tube about 80 cm. long and a centimetre 

 in diameter. It is bent into the form of a letter J and is furnished 

 with a bulb blown upon the short arm. The capacity of the bulb 

 is approximately the same as that of the long arm. The tube, 



