THE PLANT 79 



slight diffusion, whereby some of the dissolved matters in the 

 soil water enters, while some of the dissolved matter of the 

 plant juice leaves the plant, though at the same time, because 

 of its approximately semi-permeable character, osmotic pressure 

 is set up, owing to the liquid within being more concentrated 

 in dissolved matter (though of different nature) than the 

 liquid without. This latter effect is evident in the root- 

 po-essure which is shown by plants, and which causes the sap 

 to escape when a stem is cut. In some cases the magnitude 

 of this root-pressure has been measured, and found to amount 

 to two or three atmospheres. 



Hence by diffusion through the roots and root-hairs of a 

 plant the mineral matter and nitrates are taken in and forced 

 mainly by osmotic pressure (set up, not by these inorganic 

 substances, but by the sugar and other carbonaceous consti- 

 tuents of the sap) into the stem and leaves. At the same 

 time a portion of the acid juice probably escapes by diffusion 

 from the root-hairs and exerts an important solvent action 

 upon insoluble phosphates and potash compounds present in 

 the soil.* Some of the dissolved phosphates, tkc, can then 

 enter the plant again by diffusion. 



The Stem may here be regarded as the mere means of 

 communication between the roots and the leaves. It, however, 

 often plays other parts, in some cases acting as a storehouse 

 for reserve materials or for useless matters taken in by the 

 plant. 



The Leaves perform a very important part in the chemical 



processes of plant-life. It is through the leaves that the 



assimilation of carbonaceous matters takes place, and probably 



the albuminoids and amides are here formed out of the carbon 



compounds and the nitrates, phosphates, and sulphates taken 



in by the roots. Another important function of the leaves is 



* As stated in the footnote on page 41, it is now believed that carbon 

 dioxide is the main agent tending to bring into solution the constituents 

 of the soil which are insoluble in water, and that the acxd juices referred 

 to play very little, if any, part in the process, 



