43 



and the tendrils of climbing plants, from 

 the light, we might almost attribute the 

 upright growth of the stem to the mere 

 swelling (turgescence) of the cellular tissue 

 of the new green shoot, and its aberration 

 from the perpendicular to the effect of 

 light on the swelling. 



It is asserted, on microscopic observa- 

 tion, that the bark of all new green shoots 

 is entirely composed of two layers of cel- 

 lular tissue. The cells of the outside layer 

 of tissue decrease in size from within out- 

 wards ; the cells of the inside layer de- 

 crease 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 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 



