68 FLOW OF THE SAP. 



or ; thus growing inwardly from the exterior, and therefore endogenous. 

 Such continue to grow in height without increasing their diameter, as 

 we find in the long canes or reeds. These are of the great division 

 Mono-cotyledons, while the other division, Di-cotyledons, or exogens, 

 are the reverse of this in their growth. These divisions should be 

 remembered. In the latter the vascular system begins to organize 

 around the pith and continues to form outwardly by annual layers of 

 liber deposited by the cambium flowing downwards between it and 

 the bark. These concentric layers are seen near the base of the 

 tree, when cut. 



The circulation, then, is the passage upward of the original sap 

 or watery fluid from the roots, through the new wood beneath the 

 bark to the leaves where it is changed by the action of the leaves and 

 the air. From thence it flows downward through the liber, a part 

 passing off horizontally to the centre and depositing a portion of its 

 solid matter ; hence the oldest or heart-wood, has the greatest portion 

 of solid substance. The sap being first water, holding in solution 

 earths, salts, etc., it takes up the mucilage and other soluble matter 

 it finds in its passage and increases in density until it reaches and is 

 distributed among the leaves. The attraction of the leaves is thought 

 to be the cause of the flow of the sap. It begins to flow in the 

 spring from the ends of the branches ; but this is not the motion 

 of feap which continues in winter. This is the filling up of the sys- 

 tem effected by the attraction of the roots, and not by the exhalations 

 from the leaves. It is therefore probable that, as the leaves do not 

 exhale or attract in winter, the sap does not then flow as in summer. 



The effects of injury to the central wood or pith, are not great, it 

 being often destroyed, as we see in old trees, without affecting the 

 stem. The mere bark or rind may also be destroyed without per- 

 manent injury, but the liber and arburnum cannot be injured without 

 danger to the plant. In some cases of emergency plants will alter 

 their functions and circulate their fluids laterally instead of horizon- 

 tally. Many curious experiments have thus tested the pliancy and 

 restorative powers of plants. Girdling or ringing a tree, usually 

 destroys it ; but it may be saved from death by taking a circle or 

 section from the limb of a similar tree and adapting it carefully to the 

 wounded edges and binding them with grafting clay. The application 

 need not encompass the whole trunk, as the union by a portion will 

 preserve the whole tree, and the other part will be covered in time. 



Stems are divided into 7 classes, the caulis, or proper stem, the 

 culm, scape, peduncle petiole, frond and stipe. When plants have no 

 stems, flowers and fruits grow from the tops of roots. The caulis is 

 seen in the stem of the tree and common shrub, or annual plants. 

 The culm, or straw is the stem, as seen in grasses, grains, bamboo, 

 sugar cane, rushes, etc. It is without knots, as in the bullrush ; 

 jointed, as in Indian corn and wheat, and bent as in fox-glove and 



