145 



CHAPTER XII. 



THE MOVEMENT OF WATER IN THE PLANT 

 TISSUES. 



In the tissues of a living plant there is a continual movement 

 of water, the amount of movement being governed by various 

 circumstances. This movement may be readily described under 

 three heads. 



1. Slow Movement. — Wherever cells are growing water is 

 needed to aid their growth. Again, during the process of 

 assimilation, water is the chief source of the hydrogen which 

 is required for the formation of the organic materials. Also 

 water is needed in the reservoirs where the assimilated food is 

 stored up in order to re-dissolve that food and convey it to the 

 growing parts. All of these necessities cause a flow of water 

 towards these parts. This flow, like the growth and assimila- 

 tion, is slow. It is caused by osmosis, the growing cell ab- 

 sorbing water from the next, and so on till the source of water 

 is reached. It takes place in the parenchyma and meristem of 

 the stem. It is greatest during the spring and early summer — 

 the time of most rapid growth. 



2. Eapid Movement. — In plants which grow entirely under 

 water the slow movement is the only one which exists. In all 

 ordinary plants there is, however, another kind of movement 

 going on due to the transpiration which we have seen takes 

 place in the leaves. As water thus escapes a fresh supply has 

 to be obtained, and this passes up entirely through the lignified 

 cells of the xylem. If the supply is not sufi5cient to replace 

 the loss, the leaves droop and wither. This is the reason why a 

 branch cut from a tree so soon withers, as there is no water to 

 compensate for evaporation. If such a branch is placed in 



