74 BOTANY. 



amount of water equal to a layer 33 mm. (about 1^ in.) 

 deep over all its loaf-surface. When we remember that 

 the usual evaporation from a water-surface for the same 

 period is from 500 to 600 or more millimetres (20 to 25 in.) 

 we must conclude that leaves, instead of being organs for 

 increasing evaporation, are able to successfully resist evapo- 

 ration. 



143. The Movement of Water in the Plant. — It is clear, 

 from what 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, however, be shown by cutting off a leafy shoot at a 

 time when evaporation 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 norinal condition. If in such an experiment 

 a colored watery solution (as of the juice of Poke-berries) 

 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. 



144. As would readily be surmised, the tissues in ordi- 

 nary plants which appear to be the best conductors are 

 those composed of elongated wood-cells, and it is doubt- 

 less through them that the greater part of the water passes. 

 Furthermore, it is probable that the movement of the water 

 is through the substance of the cell-walls, and not; at least 

 to any great extent, through the cell-cavities. According 



