242 



ELEMENTARY STUDIES IN BOTANY 



becomes as closely related to the activities of the stock plant 

 as are the ordinary branches. The scions are usually cut 

 in the fall, after the leaves have fallen, are kept through 

 the winter in moist soil or sand, and the grafting is done 

 in the spring. A number of important things are secured 

 by grafting, but chief among them is the propagation of 

 useful varieties with certainty and with a great saving of 

 time, as compared with their propagation from the seed. 



In Monocotyledons the vascularJpundles of the stem are 

 not arranged so as to form a(ftyillow cylinder, but are 

 more or less irregularly scattered, as may be seen in a 



cross-section of a corn-stalk (Fig. 

 210). As a consequence, there 

 is no inclosing of a definite pith, 

 nor is there any distinctly bounded 

 cortex. In the bundles there is no 

 cambium, and therefore new w 

 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 



!7'1? 



FK, 210.- cross andTongitudi- branching and no increased foliage 

 nai sections of a cornstalk, from year to year. A palm well 



showing the scattered vas- ... ,. , , . . , . 



cuiar bundles. illustrates this habit, with its co- 



lumnar, unbranching trunk, and its 



crown of leaves, which continue about the same in number 

 each year. 



134. 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 ex- 

 planations of the ascent of sap in trees have been suggested, 



