372 HOW CROPS GROW. 



is either pumped into the plant, so to speak, by the root- 

 power already noticed (p. 269), or it is generated in 

 the trunk itself. The water contained in the stem in 

 winter is undoubtedly that raised from the soil in the 

 autumn. That which first flows from an auger-hole, in 

 March, may be simply what was thus stored in the trunk ; 

 but, as the escape of sap goes on for 14 to 20 days at the 

 rate of several gallons per day from a single tree, new 

 quantities of water must be continually supplied. That 

 these are pumped in from the root is, at first thought, 

 difficult to understand, because, as we have seen (p. 272), 

 the root-power is suspended by a certain low tempera- 

 ture (unknown in case of the maple), and the flow of 

 sap often begins when the ground is covered with one or 

 two feet of snow, and when we cannot suppose the soil 

 to have a higher temperature than it had during the pre- 

 vious winter months. Nevertheless, it must be that the 

 deeper roots are warm enough to be active all the winter 

 through, and that they begin their action as soon as the 

 trunk acquires a temperature sufficiently high to admit 

 the movement of water in it. That water may be pro- 

 duced in the trunk itself to a slight extent is by no 

 means impossible, for chemical changes go on there in 

 spring-time with much rapidity, whereby the sugar of 

 the sap is formed. These changes have not been suffi- 

 ciently investigated, however, to prove or disprove the 

 generation of water, and we must, in any case, assume 

 that it is the root-power which chiefly maintains a pres- 

 sure of liquid in the tree. 



The issue of sap from the maple tree in the sugar- 

 season is closely connected with the changes of tempera- 

 ture that take place above ground. The sap begins to 



of trees, varies considerably in different seasons of the year, ranging, 

 in case of the beech, from 3D to 4!i per cent of tlie fresh-felled tree. The 

 greatest proportion of water in the wood was found in the months of 

 December and January; in the bark, in March to May. The minimum 

 of water in the. wood 'occurred in May. June, and J'uly: in the. bark, 

 much irregularity was observed. Chvni. Ai-kcrsmann, 1866, p. 159. 



