CHEMI8TBT AND PHTSICS OF PLANTS. 63 



126. This avidity of protoplasm for water plays an im- 

 portant part in the general economy of the plant. By it 

 all the cells which contain protoplasm are kept turgid, and 

 by the tension thus created the soft parts of plants are 

 made rigid. It plays no small part also in keeping up the 

 supply of moisture in living tissues when wasted by evapo- 

 ration. 



127. Water in the Cell-Walls. — According to Nageli's the- 

 ory, the wall of the cell is not a membrane which separates 

 the water of one cell-cavity from that in the next, but rather 

 a pervious stratum, composed of solid particles (molecules) 

 which are not in contact, and between which the water 

 freely passes. In a living tissue the water is continuous 

 from cell to cell, and constantly tends to be in equilibrium 

 — i.e., the turgidity of the cells is approximately equal 

 throughout the tissue, and likewise the wateriness of both 

 cell-walls and cell-contents. 



128. In the simpler aquatic plants the water of the cells 

 and their walls is continuous with that in which they grow. 

 Likewise the water in the tissues of roots or other absorb- 

 ing organs of the higher aquatic plants is continuous with 

 that which surrounds them ; and even in ordinary terres- 

 trial plants there is a perfect continuity of the water in the 

 root-tissues with the moisture of the soil. 



129. The Equilibrium of the Water in the Plant.— The 

 water in the tissues of every plant tends constantly to be- 

 come in equilibrium, and this state would soon be reached 

 were it not for certain disturbing causes which are almost 

 as constantly in action. In any cell an equilibrium may 

 soon be reached between the two forces which reside re- 

 spectively in the cell-wall and the protoplasm, viz., (l) the 

 attraction of the surfaces of the molecules for the water, 



