70 WORK OF THE STEM 



the bark. Its course is undoubtedly for the most part through 

 the sieve tubes (Figs. 63, 64), which are admirably adapted to 

 convey liquids. In addition to these general upward and down- 

 ward movements of sap, there must be local transfers laterally 

 through the stem, and these are at times of much importance 



to the plant. 



Since the liquid building material travels straight down the 



stem, that side of the stem on which the manufacture of such 



material is going on most rapidly 

 should grow fastest. Plant food is 

 made out of the raw materials by the 

 leaves, and so the more leafy side of a 

 tree forms thicker rings than the less 

 leafy side, as shown in Fig. 76. 



89. Rate of movement of water in 

 the stem. There are many practical 



FIG. 76. Unequal growth of difficulties in the way of ascertaining 



rings of wood in a nearly exactly how fast the watery sap travels 

 horizontal stem of juniper frQm the rQot tQ the leayeg lfc ^ how _ 



ever, easy to illustrate experimental! y 



the fact that it does rise, and to give an approximate idea of 

 the time required for its ascent. The best experiment for be- 

 ginners is one which deals with an entire plant under natural 

 conditions ; that is, by allowing a plant to wilt from lack of 

 water, then watering it freely and noting how soon the leaves 

 begin to recover their natural appearance and positions. 



The interval of time will give a very rough idea of the time 

 of transfer of water through the roots and the stem of the plant. 

 From this, by measuring the approximate distance traveled, a 

 calculation could be made of the number of inches per minute 

 that water travels in this particular kind of plant, through a 

 route which is partly roots, partly stem, and partly petiole. 

 Still another method is to immerse the cut ends of leafy stems 

 in eosin solution and note carefully the rate of ascent of the 

 coloring liquid. This plan is likely to give results that are too 



