820 ASCENT OF THE SAP. 
leaves, &c. (as indicated by the arrows in the figure), to be 
aerated and elaborated. After this it is returned to the stem; 
and descends probably by the inner bark and cambium layer of 
Dicotyledons towards the roots from which it started (page 825) ; 
and by means of the medullary rays and the general permea- 
bility of the tissues of which plants are composed, it is also 
distributed to the different parts where new tissues are being 
formed, and where the secretions are to be deposited. This 
general distribution of the fluid matters through the plant is 
commonly termed the Circulation of the Sap. The fluid as it 
ascends is called the Ascending or Crude Sap; and as it de- 
scends, the Descending or Elaborated Sap. Although the term 
Circulation is thus commonly applied to this movement of 
the sap, it must be borne in mind, that the process bears no 
analogy to the circulation of the blood in animals; for plants 
have no heart or any organ of an analogous nature to propel 
their fluid matters, nor any system of vessels in which a flow 
thus produced takes place. As Professor Johnson has well put 
it, ‘nutrient substances in the plant are not absolutely confined 
to any path, and may move in any direction. The fact that they 
chiefly follow certain channels, and move in this or that direc- 
tion, is plainly dependent upon the structure and arrangement 
of the tissues, on the sources of nutriment, and on the seat of 
growth or other action.’ 
A. Ascent of the Sap.—The sap in its ascent to the leaves 
passes principally through the young wood-cells and vessels 
(page 785), and therefore in Dicotyledons, when they are of 
any age, through the outer portion of the wood or alburnum. 
In such plants, also, we have but one main stream of ascending 
sap. In the stems of Monocotyledons and of Cormophytes the 
ascent also takes place through the unincrusted cells and vessels 
of the fibro-vascular bundles; and hence in such plants, and 
more especially in Monocotyledons, we have a number of more 
or less distinct ascending streams. In the lower Cryptogams 
or Thallophytes, which have no stems, there is no regular 
course of the sap, but the fluids may be noticed flowing in all 
directions through their celJs, and to be more especially evident 
in those parts which are of a lax nature. 
The cause of the ascent of the sap is, as Herbert Spencer has 
well expressed it, a disturbance of equilibrium creating a demand 
for liquid. This is produced mainly by the transpiration going 
on in the leaves, but also by abstraction of the sap by the grow- - 
ing tissues and by extravasation from the vessels by pressure. 
The circulation is helped by osmotic and capillary action, pro- 
bably by the movements of protoplasm in the cells (see page 
783), and also, when it occurs, by any swaying motion of the 
branches causing intermittent pressure on the vessels. In the 
winter no transpiration takes place, and the wood of the stem 
and roots is filled with watery matters holding starch and other 
