408 LECTURE XVI. 



is that pieces of stout branches, sawn off and cemented at 

 both ends, exhibit the daily periodicity of variation in bulk, 

 and further that the diameter of such pieces increases when 

 they are withdrawn from the influence of light, the temper- 

 ature being constant, and also when they are subjected to a 

 rise of temperature in darkness. In these cases the increase 

 in diameter is entirely due to the cortical parenchyma, the bulk 

 of the wood either remaining unaltered or decreasing. The 

 expansion of the cortical parenchyma, cannot be due to an in- 

 crease in the total quantity of water, for none enters the piece 

 of branch during the experiment. It is due entirely to a re- 

 distribution of the water present. This factor doubtless enters 

 largely into the production of the variations in diameter of 

 rooted plants. The wood appears to act as a reservoir (see 

 p. 99) of water upon which the cortical parenchyma can 

 draw in expansion, and to which it can return the excess of 

 water on contraction. 



These facts are of great importance, for they afford an 

 illustration of the sensitiveness of the protoplasm of cells 

 which belong neither to growing, in length at least, or to 

 motile organs, and also in that they throw light upon the 

 mechanism of growth and of movement. A full discussion of 

 them is reserved for the present, but as much may be now said 

 concerning them, that they confirm what has been said as to 

 the effect of temperature in increasing the motility of the pro- 

 toplasm, and of light in decreasing it. The significance of these 

 facts is this, that as the motility of the protoplasm of the 

 parenchymatous cells increases under the influence of a higher 

 temperature, the vacuoles increase in size and the cells take up 

 more water ; and conversely, that as the motility of the pro- 

 toplasm diminishes under the influence of light, the paren- 

 chymatous cells give up water. When this takes place in a 

 growing organ, the growth of the cells must obviously be 

 correspondingly affected, and, as a matter of fact, the daily 

 period of variation in bulk and that of growth are closely 

 similar. The daily variation in bulk of the cells is then the 

 cause of the daily periodicity of tensions, and, in growing 

 organs, of the daily periodicity of growth in length. 



