STEMS 



57 



Fio. 56. A corn-stalk, show- 

 ing in cross - section and 

 longitudinal section the 

 scattered vascular bundles. 



a hollow cylinder, but are more or less irregularly scattered, 

 as may be seen in a cross-section of a corn-stalk (Fig. 56). 

 As a consequence, there is no en- 

 closing of a definite pith, nor is 

 there any distinctly bounded cor- 

 tex. In the bundles there is no 

 cambium, and therefore new wood 

 and bast cannot be added to the 

 old, so that in the trees there is no 

 annual increase in diameter; and 

 this means that there is no branch- 

 ing and no increased foliage from 

 year to year. A palm well illus- 

 trates this habit, with its columnar, 

 unbranching trunk, and its crown 

 of leaves, which continue about the same in number each 

 year. 



25. Ascent of sap. The water entering the plant by the 

 roots and .moving upward through the stem is usually 

 called sap. It is not pure water, but contains certain soil 

 substances dissolved in it. In low plants, as most annuals, 

 the ascent of sap requires no special explanation; but in 

 plants such as trees, in which the crown of leaves is many 

 feet above the soil, the case is very different. Several 

 explanations of the ascent of sap in trees have been sug- 

 gested, and all have been disproved, so that we are as yet 

 entirely in the dark as to the method. 



That the path of ascent is through the vessels of the 

 wood, and not through cortex or bast or pith, may be 

 demonstrated by a simple experiment. A stem of corn 

 or sunflower or balsam is cut off and placed in water for 

 an hour. Then it is transferred to a vessel containing 

 water stained with cheap red ink (a solution of eosin), 

 and exposed to diffuse light. A few hours later, sections 

 of the stem will show the wood vessels stained red, the 



