Chai>. TIL] DOWNWARD GROWTH OF THE ROOTS. 
91 
layer decrease in size from without inwards. 
Thus the largest cells of each layer are next one 
another, or in the middle of the bark, and the 
smallest cells are on the two sides; that is, the 
outside and the inside of the bark. Owing to 
this formation, swelling would stretch a slip of 
bark perfectly straight, since each layer of tissue 
would tend to curve itself outward. But in 
plants which grow towards the light the inside 
layer of tissue is the thickest, and therefore the 
most powerful in its action ; consequently swelling 
would bend a slip of such bark inwards or 
towards its shoot, and such a shoot would be 
held up by the inward pressure of its bark all 
round it, as a wall may be propped from both 
sides. But in the bark of plants, or parts of 
plants, which grow from the light, such as ivy, 
the tendrils of climbing plants, &c., the outside 
layer of tissue is the strongest; and the tops of 
the shoots of such plants tend to stand upright 
by the pulling outward of their bark from all 
sides, like the mast of a ship. But light, by 
increasing the giving off of water (transpiration), 
decreases swelling; and when it falls unequall] 
on plants, the forces on the illumined side wil 
be weakened, and the shoot, according to the 
formation of its cellular tissue, will be turned 
How turges- 
cence acts on 
the cellular for¬ 
mation. 
How light acts 
on this tur- 
ge see nee. 
