260 TRANSFER OF WATER THROUGH THE PLANT. 



695. The two objections to the first method are : (1) that the 

 protoplasmic body of the cell resists the entrance of nearly all 

 coloring-matters, therefore with many dyes it is necessary to 

 experiment with cut stems and branches, allowing the dye to 

 enter at the cut surface ; but, as will be shown later, a cut sur- 

 face which has been exposed to the air, even for an instant, 

 loses part of its power of absorbing water ; (2) it is by no means 

 certain that the dye passes through the stem as rapidly as the 

 water in which it is dissolved. That it does not, seems more 

 than probable from the simple experiment of suspending one 

 end of a strip of filter-paper in a solution of any dye ; the water 

 will rise faster than the dye, and form a moist space above that 

 part of the paper which becomes colored. 



696. The second method of experimenting is based upon 

 the ease with which certain chemical substances foreign to the 

 plant can be detected in it if once they can be introduced into 

 and carried through its tissues. Dilute solutions of salts of 

 lithium, for instance the citrate, serve best for this method, and 

 Pfitzer suggests that the}- be applied to the roots of a plant 

 which has been allowed to wilt somewhat from drought. 



697. The two objections which maybe urged against the second 

 method, are : (1) the chemical used may cause more or less dis- 

 turbance in the plant, and may even excite disordered processes, 

 and it is plain that no correct conclusions relative to the rapid- 

 ity of transfer in a healthy plant can be drawn from one which 

 is in a state of disease ; (2) the presence of a diffusible salt, for 

 instance one of lithium, may change the osmotic relations of the 

 tissues with which the salt comes in contact. But in spite of 

 these serious difficulties, these methods are of considerable use 

 when cautiously employed. 



698. The above methods indicate that the most rapid transfer 

 of water is through the lignified cell- walls of the framework of 

 the plant. The source of supply at the root furnishes the need- 

 ful amount of water to the ligneous tissues of the fibrils, and 

 these convey it to the converging bundles which constitute the 

 framework of the plant. In the leaves the framework divides and 

 subdivides to form the network of the leaf blade, and here the 

 ligneous cells and ducts are in intimate contact with the paren- 

 chyma cells which make up the pulp of the leaf. That water 

 finds its way by preference through the fibre-vascular bundles 

 even in the more delicate parts, is shown b}- placing the cut 

 peduncle of a white tulip, or other large white flower, in a harm- 

 less dye, and then again cutting off its end in order to bring a 



