204 CIRCULATION IN 



wood and bark formed the preceding year ; or the inner surface of the bark and outer surface 

 of the wood, while thus surcharged with sap, form two layers of cells, indistinct at first, in 

 consequence of an excess of transparent fluid. These cells, whether developed directly from 

 the surface of the wood and bark or in the fluids poured out, are rapidly and simultaneously 

 formed, and in due time constitute the wood — a new layer of growth. The tree increases in 

 length by the production of cells, but in breadth, the lateral growth, the cells are all equal in 

 size, and never produced within them new cells. 



CIRCULATION IN ORGANIZED BODIES. 



The nature and character of all organized bodies requires that their tissues should be tra- 

 versed by a fluid. This fluid should contain all the elements of nutrition : unless this is the 

 case, vegetables would cease to grow, and animals would perish by starvation. The fact that 

 there is a circulation is proved by occular inspection ; but occular inspection has not, as yet, been 

 competent to determine all that is maintained by physiologists with respect to it. By some, it 

 is supposed there is a double circulation ; that is, the fluid or sap rises in the stem of the 

 plant, reaches finally the leaves, where it undergoes certain changes, and thence it begins to 

 descend, till it reaches the root, having gone through the entire tissues of the plant, by an up- 

 ward and downward movement. But is this double movement proved 1 I think not. Theory 

 assumes that it is a function of the leaf to effect a change in the circulating fluid, and upon this 

 change the growth of the plant or tree depends. Facts, however, show that the sap is changed 

 in the tissue of the plant, and before it reaches the leaves. This is a function of the cell wall 

 and the fluid which it contains, being probably in part functional and in part chemical. The 

 sap passes on to the leaf, and in the spring, in temperate climates, it finds undeveloped leaves 

 in the buds of the preceding year. In the perfect development of the leaf the sap expends its 

 nutritive matter ; that is, it develops new cells, and at the same time the superfluous water is 

 exhaled from the growing and perfect leaves. By this arrangement, too, the stem grows, be- 

 ing elongated and increased in diameter. We see too, in this arrangement, the source of this 

 power, which moves the fluid ; the surface cells being deprived of a portion of their fluids by 

 exhalation, they are prepared to imbibe or receive a new supply from the contiguous and com- 

 paratively distended cells. The great extent of surface over which this exhalation goes on is 

 competent to influence the whole mass of fluids in the tissues of the plant. Those who maintain 

 the double circulation appeal to experiment : they say that where a tree has been girdled, it may 

 live, but it will grow only above the girdled zone. This is, however, an error of observation ; 

 the tree still grows below the girdle, and what is important is, when some trees are cut off, 

 they may continue to grow slowly, producing new wood around and over the extremity of 

 the stump. 



There are many anomalies in the growth and nutrition of vegetables. The possibility of 

 sustaining the life of trees by introducing its sap laterally, is also proved ; and hence the 

 fact that it may pursue a retrograde course can not be overlooked ; still the normal course and 

 direction of the sap is, probably, similar to the view which I have hinted at. In the spring a 



