80 ESSENTIALS OF BOTANY 



Examine longitudinal sections of some of the twigs, the potatoes, 

 and the roots. In drawing conclusions about the channels through 

 which the ink has risen (those through which the newly absorbed 

 soil-water most readily travels), bear in mind the fact that a slow 

 soakage of the red ink will take place in all directions, and there- 

 fore pay attention only to the strongly colored spots or lines. 



What conclusions can be drawn from this experiment as to the 

 course followed by the sap V 



From the familiar facts that ordinary forest trees appar- 

 ently flourish as well after the almost complete decay and 

 removal of their heartwood, and that many kinds will live 

 and grow for a considerable time after a ring of bark 

 extending all round the trunk has been removed, it may 

 readily be inferred that the crude sap in trees must rise 

 through some portion of the newer layers of the wood. 

 A tree girdled by the removal of a ring of sapwood 

 promptly dies. 



94. Downward Movement of Liquids. — Most dicotyle- 

 donous stems, when stripped of a ring of bark and then 

 stood in water, as shown in Fig. 47, and covered with a 

 bell-jar, develop roots only at or near the upper edge of 

 the stripped portion, ^ and this would seem to prove that 

 such stems send their building material — the elaborated 

 sap — largely at any rate down through the bark. Its 

 course is undoubtedly for the most part through the sieve- 

 cells (Fig. 42), 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. 



1 This may be made the subject of a protracted class-room experiment. 

 Strong shoots of willow should be used for the purpose. 



