Transpiration and Water Movement 109 



in size may be followed by the laying down of new veinlets 

 of a lower order (at first fewer tracheids) from each side 

 of the original space. These may be at first procambial 

 in nature, but tracheids are rapidly differentiated within. 

 It is of special interest to note that the sieve tubes disap- 

 pear relatively early in the minute continuations of the 

 bundles. 



65. Rate of transport. The rate of transport of water 

 in the fibrovascular bundles may be determined with a 

 fair degree of accuracy by means of the rise of dyestuffs 

 as before noted, but more accurately in many cases by the 

 method of Sachs, wherein lithium nitrate is used in the 

 solution and its presence after intervals determined by 

 burning the tissues and examining the flame spectro- 

 scopically. According to Sachs the rate of water rise is 

 extremely diverse, and may vary from a few centimeters 

 per hour to one or more meters. Doubtless the extremes 

 are often greater than these indicated, but unquestionably 

 the difficulties of measurement are greater at the extremes. 



LABORATORY WORK 



Indication of transpiration. Stahl's cobalt test may be 

 employed to determine water-loss from a plant surface. In- 

 cidentally it determines roughly the presence or absence of 

 stomata, or the relative abundance upon the upper and lower 

 surfaces of the leaf. Soak filter paper in a 5 per cent solution 

 of cobalt chloride, dry in the oven or over a flame, and note the 

 blue color. Breathe upon a small piece of this paper and note 

 that the absorption of moisture induces a change to pink. 

 Now cut out two pieces of the paper of equal size ; place one 

 upon the upper and one upon the lower side of the leaf to be 

 tested, cover each with a piece of mica and cement the latter 



