COMPARED WITH THOSE OP ANIMALS. 71 



minate in a network of anastomosing capillaries like those 

 in the leaves of plants, by means of which the blood is dis- 

 tributed to all parts of the organ. 



These capillaries not only cover the surface of the body 

 and all its organs, but they penetrate their substance, con- 

 veying the blood to every part of the fabric. The capillary 

 system is interjacent between the venous and arterial sys- 

 tems. It is in traversing these capillaries that the blood 

 changes from scarlet to purple, in consequence of giving up 

 its nutrient principles to the tissues. 



The blood leaves these capillaries by means of the veins, 

 each of which is first formed by the union of several capil- 

 laries; these collect and return the blood after it has 

 traversed the organs of the body, converge and reunite into' 

 larger and larger vessels, like the roots of a tree or the 

 sources of a river. In this manner the blood is again 

 brought back to its original source, the heart, from whence 

 it is driven into the lungs. There the fluid is brought into 

 immediate contact with the atmospheric air absorbed during 

 respiration, is oxygenated, and again returned to the heart, 

 from whence it is again driven into the aorta, and conveyed 

 by its ramifications as before, to all parts of the body. 



Whilst therefore in plants the capillary system is con- 

 fined to the leaves, in animals it pervades every part of the 

 body. The capillary system of plants is only required 

 during the season of vegetable activity, and is not needed 

 during the period of vegetable repose. The leaves are 

 therefore only temporary organs, and the plant loses its 

 capillaries when it becomes defoliated. The woody fasci- 

 culi which form the newly developed shoots at the extremi- 

 ties of the branches alone remain, permanently attached to 

 the plant, together with the dormant buds on their exterior 

 surface. It is through the influence of the leaves with 



