PLANT PHY8I0L0OT. 103 



supplemektaby notes qn the movement op water ik the 



Plant. 



I. The Movement of Water in the Plant. ~-lX is clear, from wlaat has 

 been said, that in many -celled plants there must be a' considerable 

 movement of water in some parts to supply the loss by evaporation 

 Thus in trees there must be a movement of water through the roots, 

 stems, and branches to the leaves, to replace the loss in the latter. 

 This is so evident that it scarcely needs demonstration ; it can, how 

 ever, be shown by cutting off a leafy shoot at a time when evapora- 

 tion is rapid ; in a short time the leaves wither and become dried up 

 unless the cut portion of the shoot be placed in a vessel of water ; in 

 the latter case the water will pass rapidly into the shoot, and the 

 leaves will retain their normal condition. If in such an experiment 

 a colored watery solution (as of the juice of Poke-beiries) be used 

 instead of pure water, it will be seen that the liquid has passed more 

 abundantly through certain tracts than through others, indicating 

 that the tissues are not equally good as conductors of watery solutions. 



II. Path of Movement. — As would readily be surmised, the tissues 

 in ordinary plants which appear to be the best conductors are those 

 composed of elongated wood-cells, and it is doubtless through them 

 that the greater part of the water passes ; furthermore, it is probable 

 that the movement of the water is mainly through the substance of 

 the cell- walls. 



III. Rapidity of Movement. — The rapidity of the upward move- 

 ment of water varies greatly in different plants and under different 

 conditions. In a silver-poplar a, rate of 23 cm. (9 in.) an hour has 

 been observed ; in a cherry-laurel 101 cm. (40 in.) ; and in a sun- 

 flower 32 metres (73 feet). 



IV. No Circulation of 8ap. — While there is an upward movement 

 of the water in plants because of the evaporation from the leaves, 

 there is no downward movement, as has been popularly supposed. 

 The "circulation of the sap," in the sense that there is an upward 

 stream in one portion of the plant and a corresponding downward 

 stream in another, does not exist. Likewise, the belief still held by 

 some people that in the autumn or early winter " the sap goes down 

 into the roots," and that " it rises " in the spring, is entirely erroneous. 

 There is actually more water (sap) in an ordinary deciduous tree in 

 the winter than there is in the spring or summer (excluding, of course, 

 the new and very watery growths). 



V. T7te Flow of Water (sap) from the stems and branches of certain 

 trees, notably from the sugar-maple, appears to be due to the quick 

 alternate expansion and contraction of the air and other gases in the 

 tissues from the quick changes of temperature. The water is forced 



