THE WORK OF STEMS 



243 



The fact that the run of sap commences in the 

 spring before the leaves appear (as in the Sugar 

 Maple, etc.) is apparently due to the formation at 

 that time of large quantities of sugar and other water- 

 attracting substances in the stem which draw up the 

 water in large quantities. 



It is a matter of common observation that 

 at certain times of the year many plants 

 bleed (i. e., exude water) when cut or 

 trimmed. Test some vigorous, well- watered 

 plants in regard to this point (especially 

 Squash, Sunflower, Dahlia, Begonia, Corn, 

 etc.) by cutting off the plant near the ground 

 and connecting the stump to a glass tube by 

 means of a section of rubber tubing (see 

 Fig. 142). Pour a little water in 'the 

 tube and support it in an upright 

 position. Place a little oil on the 

 water and mark the height at which 

 it stands. In favorable cases the 

 water will rise in the tube; this is 

 due to what is known as root -pres- 

 sure; some observed cases of root- 

 pressure show that it could, in cer- 

 tain plants, under favorable circum- 

 stances, raise the water many feet; 



J 142. Method of measuring 



but at the time of year when the amount of weed- 

 ing from a stump 



transpiration is greatest little or no (root pressure). 



