4 8 



Elementary Plant Physiology. 



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 



Fig. 25. Osmometer. A, 

 water ; , 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 E by a piece of rubber 

 tubing at G. 



