LEAF STRUCTURE. 175 



produce starch and other carbohydrates, as oils, sugars, 

 gums, etc. These are either used to supply the plant's 

 immediate wants or stored in some of its organs for 

 future use. 



The decomposition of water and carbon dioxide lib- 

 erates oxygen, which may be seen in bubbles on the 

 submerged parts of water plants ; this gas escaping 

 into the air, helps to keep it pure. 



431. Movements of fluids. The root takes up from 

 the earth the watery substances which are presented 

 to it ; the cells at the extremities of the root and 

 rootlets are first gorged ; these impart to the cells and 

 vessels next in contact, which take up the fluids by in- 

 filtration, and so they are passed on up the stem largely 

 through the cells and vessels of the last season's wood, 

 and outward through the same class of cells and 

 ducts, along the branches to the leaves and new twigs. 

 Having reached these green parts, much of the water 

 passes off by evaporation ; what remains becomes 

 changed by the action of sunlight and fitted for build- 

 ing up the plant's structure. It then by some mode 

 of transfusion finds its way back to all the growing 

 parts of the plant where new material is needed. 



432. Circulation. Careful observation and experi- 

 ment have demonstrated that there is an upward cur- 

 rent of water or watery fluids through the stem, by 

 way principally of the fibre-vascular tissues ; but no 

 downward movement has been detected answering to 

 a current. Hence there is not a circulation which cor- 

 responds to what takes place in the higher animals. 

 Yet the prepared sap reaches parts of the plant's 

 structure lower than the points where it was prepared ; 

 hence it must go downward, 



