l6o THE STEM PROPER 



The ducts are there, but they are formed differently from 

 those of other exogens, and can not be studied without a 

 compound microscope. From what part of the stem does 

 the rosin exude ? Place a cutting in red ink and notice 

 through what part the fluid rises ; where, would you judge 

 from this, is the most active part of the stem ? 



PRACTICAL QUESTIONS 



1. Explain the principle upon which boys slip the bark from certain 

 kinds of wood in spring to make whistles. (220.) 



2. Why can not they do this in autumn or winter? 



3. Name some of the plants commonly used for this purpose. 



4. Is the .spring, after the buds begin to swell, a good time to prune 

 fruit trees and hedges? Why? (220.) 



5. What is the best time, and why? 



6. Why are grape vines liable to bleed to death if pruned too late in 

 spring? (220, 221.) 



7. Why are nurserymen, in grafting, so careful to make the cambium 

 layer of the graft hit that of the stock? (220.) 



8. In calculating the age of a tree or bough from the rings of annual 

 growth should we take a section from near the tip, or the base? Why? 

 (224.) 



MOVEMENT OF WATER THROUGH THE STEM 



Material. — An egg, a small cup, and some salt water. A potted 

 young plant of corn, calla lily, tropaeolum, sunflower, etc. A few centi- 

 meters each of glass tubing and rubber tubing about the diameter of the 

 stem of the plant. A twig of willow, currant, or other easily rooting 

 shrub. 



226. Difficulty of Accounting for Sap Movement. — Just 

 what causes the rise of sap in the stem is one of the 

 puzzles of vegetable physiology that botanists have not 

 yet been able to solve completely. It is closely connected 

 with the phenomena of transpiration, the rapidity of the 

 current increasing and decreasing according to the activity 

 of the evaporating surfaces. If loss of water begins at any 

 spot through growth or transpiration, the nearest tissues 

 give up their water first, then the more remote, and so on, 

 till the most distant — generally the roots — have to absorb 

 water from without, and thus a constant current is kept up 

 toward the places where moisture is needed. 



