48 



Elementary Plant Physiology. 



E 



D 



lifeless walls remain, becoming a part of the machinery of the 

 plant, under more or less direct control of the living part of the 

 organism. Thus a tree consists of a trunk made up largely of 

 a central cylinder of dead woody 

 cells surrounded by a layer of living 

 cells under the bark, usually not more 

 than a few millimeters in thickness, 

 and enclosed by the dead or dying 

 bark. 



The walls enclosing the living pro- 

 toplasts are devoid of openings large 

 enough for the passage of any appre- 

 ciable solid body, hence substances 

 entering the living cells, or being 

 thrown out by them, must be in liquid 

 or gaseous condition. A leafy-stemmed 

 plant may be regarded, from a purely 

 physical point of view, as a cylinder 

 with walls of parchment ; the lower 

 end of the cylinder extends in a great 

 number of minute ramifications in the 

 roots, and the upper end is divided into 

 branches bearing the leaves. The 

 roots are in contact with solutions in 

 the soil, and the leaves are immersed 

 in the gases of the atmosphere. 



Exchange of fluids between the 

 roots and the soil solutions, and be- 

 tween the leaves and the atmosphere, 

 is taking place constantly by osmose. 

 As a consequence, two constant 

 streams are found in the plant; one 

 from the roots to the leaves, and an- 

 other from the leaves to the roots, in 



SBr 



I- ^ --' 



Fig. 25.— Osmometer. A^ 

 water ; B^ cylinder of dialy- 

 zer tubing containing salt 

 solution; C, perforated 

 stopper ; Z), short section of 

 glass tubing ■which is con- 

 nected with a long capillary 

 tube £ by a piece of rubber 

 tubing at G, 



