244 



ELEMENTARY STUDIES IN BOTANY 



found to reach about twenty feet an hour. It is estimated 



that in ordinary broad-leaved trees the rate is probably 



three to six feet an hour. 



If certain stems are cut off near the ground, it is observed 



that after a short time the sap begins to ooze out, a phenome- 

 non that is often called " bleeding." 

 In some woody plants, as grape-vines 

 and birches, the sap flows out with 

 considerable force, indicating some 

 pressure below, which is called root- 

 pressure. While root-pressure may 

 force the sap into the stem, it is 

 entirely inadequate to force it to the 

 top of a tree. 



The so-called maple sap obtained 

 from the sugar maple is an interesting 

 illustration of the use of sap that ac- 

 cumulates in a woody stem in the 

 spring. At that time the water has 

 no opportunity to escape through leaf 

 transpiration ; so the wood becomes 

 gorged with sap, which can be drawn 

 off by boring into the wood and in- 

 serting spiles. The characteristic sugar 

 has been obtained by the sap from 



FIG. 211. Scarlet runner ... . i i 



bean marked with a scale f OOCl stored in the stem, notably 111 

 of five millimeter inter- ,1 i i j 



the older wood. 



135. Growth in length. Growth 



vals and photographed 

 after forty-eight hours ; 

 the lines closest together 



show the original spac- in length begins at the tip of the 

 stem by the formation of new cells, 



which are organized into the alternating nodes and internodes. 

 When these regions are first formed, the internodes are very 

 short, and their subsequent elongation, separating the nodes, is 

 the chief cause of the lengthening of the stem. Internodes 

 are able to elongate for only a certain time, so that the 



