GROWTH 189 



Tissues consisting of very thin cell -walls and liquids 

 would not by themselves be rigid. It is only when 

 cells become overfilled with liquid till their walls 

 distend, and the inner tissues press upon the outer ones 

 and themselves are compressed, that an organ becomes 

 turgid, and does not easily bend over or droop as do 

 fading stems in which, owing to an insufficiency of water, 

 the tension of the membranes in individual cells as 

 well as the mutual tension of the tissues is weakened. 



Let us make a comparison, rather rough it is true, 

 but one which will give us a general idea of what takes 

 place in the plant during its growth. I hold a glove 

 in my hand. Its empty fingers hang down. I breathe 

 air into one of the fingers and seize it near its base, — 

 now it can preserve the vertical as well as the horizontal 

 position without drooping, or bending. This finger 

 filled with air represents to a certain extent a cell over- 

 filled with sap or a stem, the external parts of which 

 remain stiff under the outward pressure of its more 

 rapidly growing inner parts. 



Let us now discover the relation between all that has 

 so far been explained and our original question : Why 

 is it that a stem laid in a horizontal position curves 

 upwards by itself ? 



Whilst a stem remains in a vertical position, the force 

 of gravity acts uniformly on all its parts ; but as soon 

 as we place it in a horizontal position, the conditions 

 change. Owing either to a stronger current of the 

 nutrient substances or to other causes, the lower part 

 will grow and stretch more quickly than the upper. 

 We already know that the tendency of the inner 

 part of the stem to stretch is continually checked by 

 the resistance of the epidermis outside. But in the 

 horizontal stem the lower half of this inner part will 

 grow more quickly and will at the same time stretch 

 the epidermis unequally — stretching the lower and 

 nearer part more strongly than the upper and more 



